ceph: fix ceph_mdsc_build_path to not stop on first component

When ceph_mdsc_build_path is handed a positive dentry, it will return a
zero-length path string with the base set to that dentry.  This is not
what we want.  Always include at least one path component in the string.

ceph_mdsc_build_path has behaved this way for a long time but it didn't
matter until recent d_name handling rework.

Fixes: 964fff74 ("ceph: use ceph_mdsc_build_path instead of clone_dentry_name")
Signed-off-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: "Yan, Zheng" <zyan@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>

fs/ceph/mds_client.c

@@ -2121,9 +2121,10 @@ char *ceph_mdsc_build_path(struct dentry *dentry, int *plen, u64 *pbase,
 		if (inode && ceph_snap(inode) == CEPH_SNAPDIR) {
 			dout("build_path path+%d: %p SNAPDIR\n",
 			     pos, temp);
-		} else if (stop_on_nosnap && inode &&
+		} else if (stop_on_nosnap && inode && dentry != temp &&
 			   ceph_snap(inode) == CEPH_NOSNAP) {
 			spin_unlock(&temp->d_lock);
+			pos++; /* get rid of any prepended '/' */
 			break;
 		} else {
 			pos -= temp->d_name.len;