RDMA: Replace zero-length array with flexible-array member

The current codebase makes use of the zero-length array language
extension to the C90 standard, but the preferred mechanism to declare
variable-length types such as these ones is a flexible array member[1][2],
introduced in C99:

struct foo {
        int stuff;
        struct boo array[];
};

By making use of the mechanism above, we will get a compiler warning
in case the flexible array does not occur last in the structure, which
will help us prevent some kind of undefined behavior bugs from being
inadvertently introduced[3] to the codebase from now on.

Also, notice that, dynamic memory allocations won't be affected by
this change:

"Flexible array members have incomplete type, and so the sizeof operator
may not be applied. As a quirk of the original implementation of
zero-length arrays, sizeof evaluates to zero."[1]

This issue was found with the help of Coccinelle.

[1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
[2] https://github.com/KSPP/linux/issues/21
[3] commit 7649773293 ("cxgb3/l2t: Fix undefined behaviour")

Link: https://lore.kernel.org/r/20200213010425.GA13068@embeddedor.com
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com> # added a few more

include/rdma/ib_fmr_pool.h

@@ -73,7 +73,7 @@ struct ib_pool_fmr {
 	int                 remap_count;
 	u64                 io_virtual_address;
 	int                 page_list_len;
-	u64                 page_list[0];
+	u64                 page_list[];
 };
 
 struct ib_fmr_pool *ib_create_fmr_pool(struct ib_pd             *pd,

include/rdma/ib_verbs.h

@@ -1876,7 +1876,7 @@ struct ib_flow_eth_filter {
 	__be16	ether_type;
 	__be16	vlan_tag;
 	/* Must be last */
-	u8	real_sz[0];
+	u8	real_sz[];
 };
 
 struct ib_flow_spec_eth {
@@ -1890,7 +1890,7 @@ struct ib_flow_ib_filter {
 	__be16 dlid;
 	__u8   sl;
 	/* Must be last */
-	u8	real_sz[0];
+	u8	real_sz[];
 };
 
 struct ib_flow_spec_ib {
@@ -1915,7 +1915,7 @@ struct ib_flow_ipv4_filter {
 	u8	ttl;
 	u8	flags;
 	/* Must be last */
-	u8	real_sz[0];
+	u8	real_sz[];
 };
 
 struct ib_flow_spec_ipv4 {
@@ -1933,7 +1933,7 @@ struct ib_flow_ipv6_filter {
 	u8	traffic_class;
 	u8	hop_limit;
 	/* Must be last */
-	u8	real_sz[0];
+	u8	real_sz[];
 };
 
 struct ib_flow_spec_ipv6 {
@@ -1947,7 +1947,7 @@ struct ib_flow_tcp_udp_filter {
 	__be16	dst_port;
 	__be16	src_port;
 	/* Must be last */
-	u8	real_sz[0];
+	u8	real_sz[];
 };
 
 struct ib_flow_spec_tcp_udp {
@@ -1959,7 +1959,7 @@ struct ib_flow_spec_tcp_udp {
 
 struct ib_flow_tunnel_filter {
 	__be32	tunnel_id;
-	u8	real_sz[0];
+	u8	real_sz[];
 };
 
 /* ib_flow_spec_tunnel describes the Vxlan tunnel
@@ -1976,7 +1976,7 @@ struct ib_flow_esp_filter {
 	__be32	spi;
 	__be32  seq;
 	/* Must be last */
-	u8	real_sz[0];
+	u8	real_sz[];
 };
 
 struct ib_flow_spec_esp {
@@ -1991,7 +1991,7 @@ struct ib_flow_gre_filter {
 	__be16 protocol;
 	__be32 key;
 	/* Must be last */
-	u8	real_sz[0];
+	u8	real_sz[];
 };
 
 struct ib_flow_spec_gre {
@@ -2004,7 +2004,7 @@ struct ib_flow_spec_gre {
 struct ib_flow_mpls_filter {
 	__be32 tag;
 	/* Must be last */
-	u8	real_sz[0];
+	u8	real_sz[];
 };
 
 struct ib_flow_spec_mpls {

include/rdma/opa_vnic.h

@@ -75,7 +75,7 @@
 struct opa_vnic_rdma_netdev {
 	struct rdma_netdev rn;  /* keep this first */
 	/* followed by device private data */
-	char *dev_priv[0];
+	char *dev_priv[];
 };
 
 static inline void *opa_vnic_priv(const struct net_device *dev)

include/rdma/rdmavt_mr.h

@@ -85,7 +85,7 @@ struct rvt_mregion {
 	u8  lkey_published;     /* in global table */
 	struct percpu_ref refcount;
 	struct completion comp; /* complete when refcount goes to zero */
-	struct rvt_segarray *map[0];    /* the segments */
+	struct rvt_segarray *map[];    /* the segments */
 };
 
 #define RVT_MAX_LKEY_TABLE_BITS 23

include/rdma/rdmavt_qp.h

@@ -191,7 +191,7 @@ struct rvt_swqe {
 	u32 ssn;                /* send sequence number */
 	u32 length;             /* total length of data in sg_list */
 	void *priv;             /* driver dependent field */
-	struct rvt_sge sg_list[0];
+	struct rvt_sge sg_list[];
 };
 
 /**