libceph: update ceph_msg_header structure

2 bytes of what was reserved space is now used by userspace for the
compat_version field.

Signed-off-by: John Spray <john.spray@redhat.com>
Reviewed-by: Sage Weil <sage@redhat.com>

include/linux/ceph/msgr.h

@@ -152,7 +152,8 @@ struct ceph_msg_header {
 			     receiver: mask against ~PAGE_MASK */
 
 	struct ceph_entity_name src;
-	__le32 reserved;
+	__le16 compat_version;
+	__le16 reserved;
 	__le32 crc;       /* header crc32c */
 } __attribute__ ((packed));