ceph: support getting ceph.dir.rsnaps vxattr

Add support for grabbing the rsnaps value out of the inode info in
traces, and exposing that via ceph.dir.rsnaps xattr.

Signed-off-by: Yanhu Cao <gmayyyha@gmail.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
diff --git a/fs/ceph/mds_client.c b/fs/ceph/mds_client.c
index 73ecb7d..5c50f79 100644
--- a/fs/ceph/mds_client.c
+++ b/fs/ceph/mds_client.c
@@ -176,6 +176,13 @@ static int parse_reply_info_in(void **p, void *end,
 			memset(&info->snap_btime, 0, sizeof(info->snap_btime));
 		}
 
+		/* snapshot count, remains zero for v<=3 */
+		if (struct_v >= 4) {
+			ceph_decode_64_safe(p, end, info->rsnaps, bad);
+		} else {
+			info->rsnaps = 0;
+		}
+
 		*p = end;
 	} else {
 		if (features & CEPH_FEATURE_MDS_INLINE_DATA) {
@@ -214,7 +221,7 @@ static int parse_reply_info_in(void **p, void *end,
 		}
 
 		info->dir_pin = -ENODATA;
-		/* info->snap_btime remains zero */
+		/* info->snap_btime and info->rsnaps remain zero */
 	}
 	return 0;
 bad: