ceph: reorder fields in 'struct ceph_snapid_map'

Group some variables based on their sizes to reduce holes.
On x86_64, this shrinks the size of 'struct ceph_snapid_map' from 72 to 64
bytes.

When such a structure is allocated, because of the way memory allocation
works, when 72 bytes were requested, 96 bytes were allocated.

So, on x86_64, this change saves 32 bytes per allocation and has the
structure fit in a single cacheline.
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>

fs/ceph/mds_client.h

@@ -355,8 +355,8 @@ struct ceph_snapid_map {
 	struct rb_node node;
 	struct list_head lru;
 	atomic_t ref;
-	u64 snap;
 	dev_t dev;
+	u64 snap;
 	unsigned long last_used;
 };