libceph: enable large, variable-sized OSD requests

Turn r_ops into a flexible array member to enable large, consisting of
up to 16 ops, OSD requests.  The use case is scattered writeback in
cephfs and, as far as the kernel client is concerned, 16 is just a made
up number.

r_ops had size 3 for copyup+hint+write, but copyup is really a special
case - it can only happen once.  ceph_osd_request_cache is therefore
stuffed with num_ops=2 requests, anything bigger than that is allocated
with kmalloc().  req_mempool is backed by ceph_osd_request_cache, which
means either num_ops=1 or num_ops=2 for use_mempool=true - all existing
users (ceph_writepages_start(), ceph_osdc_writepages()) are fine with
that.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>

include/linux/ceph/osd_client.h

@@ -43,7 +43,8 @@ struct ceph_osd {
 };
 
 
-#define CEPH_OSD_MAX_OP	3
+#define CEPH_OSD_SLAB_OPS	2
+#define CEPH_OSD_MAX_OPS	16
 
 enum ceph_osd_data_type {
 	CEPH_OSD_DATA_TYPE_NONE = 0,
@@ -139,7 +140,6 @@ struct ceph_osd_request {
 
 	/* request osd ops array  */
 	unsigned int		r_num_ops;
-	struct ceph_osd_req_op	r_ops[CEPH_OSD_MAX_OP];
 
 	/* these are updated on each send */
 	__le32           *r_request_osdmap_epoch;
@@ -175,6 +175,8 @@ struct ceph_osd_request {
 	unsigned long     r_stamp;            /* send OR check time */
 
 	struct ceph_snap_context *r_snapc;    /* snap context for writes */
+
+	struct ceph_osd_req_op r_ops[];
 };
 
 struct ceph_request_redirect {