ceph: canonicalize server path in place

syzbot reported that 4fbc0c711b ("ceph: remove the extra slashes in
the server path") had caused a regression where an allocation could be
done under a spinlock -- compare_mount_options() is called by sget_fc()
with sb_lock held.

We don't really need the supplied server path, so canonicalize it
in place and compare it directly.  To make this work, the leading
slash is kept around and the logic in ceph_real_mount() to skip it
is restored.  CEPH_MSG_CLIENT_SESSION now reports the same (i.e.
canonicalized) path, with the leading slash of course.

Fixes: 4fbc0c711b ("ceph: remove the extra slashes in the server path")
Reported-by: syzbot+98704a51af8e3d9425a9@syzkaller.appspotmail.com
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>

fs/ceph/super.h

@@ -91,7 +91,7 @@ struct ceph_mount_options {
 
 	char *snapdir_name;   /* default ".snap" */
 	char *mds_namespace;  /* default NULL */
-	char *server_path;    /* default  "/" */
+	char *server_path;    /* default NULL (means "/") */
 	char *fscache_uniq;   /* default NULL */
 };