bpf, xdp: allow to pass flags to dev_change_xdp_fd

Add an IFLA_XDP_FLAGS attribute that can be passed for setting up
XDP along with IFLA_XDP_FD, which eventually allows user space to
implement typical add/replace/delete logic for programs. Right now,
calling into dev_change_xdp_fd() will always replace previous programs.

When passed XDP_FLAGS_UPDATE_IF_NOEXIST, we can handle this more
graceful when requested by returning -EBUSY in case we try to
attach a new program, but we find that another one is already
attached. This will be used by upcoming front-end for iproute2 as
well.

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/uapi/linux/if_link.h

@@ -876,10 +876,14 @@ enum {
 
 /* XDP section */
 
+#define XDP_FLAGS_UPDATE_IF_NOEXIST	(1U << 0)
+#define XDP_FLAGS_MASK			(XDP_FLAGS_UPDATE_IF_NOEXIST)
+
 enum {
 	IFLA_XDP_UNSPEC,
 	IFLA_XDP_FD,
 	IFLA_XDP_ATTACHED,
+	IFLA_XDP_FLAGS,
 	__IFLA_XDP_MAX,
 };