ceph: mount non-default filesystem by name

To mount non-default filesytem, user currently needs to provide mds
namespace ID. This is inconvenience.

This patch makes user be able to mount filesystem by name. If user
wants to mount non-default filesystem. Client first subscribes to
fsmap.user. Subscribe to mdsmap.<ID> after getting ID of filesystem.

Signed-off-by: Yan, Zheng <zyan@redhat.com>
diff --git a/fs/ceph/super.h b/fs/ceph/super.h
index 5aa3158..9e82e29 100644
--- a/fs/ceph/super.h
+++ b/fs/ceph/super.h
@@ -62,7 +62,6 @@
 	int cap_release_safety;
 	int max_readdir;       /* max readdir result (entires) */
 	int max_readdir_bytes; /* max readdir result (bytes) */
-	int mds_namespace;
 
 	/*
 	 * everything above this point can be memcmp'd; everything below
@@ -70,6 +69,7 @@
 	 */
 
 	char *snapdir_name;   /* default ".snap" */
+	char *mds_namespace;  /* default NULL */
 	char *server_path;    /* default  "/" */
 };