cifs: cope with negative dentries in cifs_get_root

The loop around lookup_one_len doesn't handle the case where it might
return a negative dentry, which can cause an oops on the next pass
through the loop. Check for that and break out of the loop with an
error of -ENOENT if there is one.

Fixes the panic reported here:

    https://bugzilla.redhat.com/show_bug.cgi?id=727927Reported-by: NTR Bentley <home@trarbentley.net>
Reported-by: NIain Arnell <iarnell@gmail.com>
Cc: Al Viro <viro@ZenIV.linux.org.uk>
Cc: stable@kernel.org
Signed-off-by: NJeff Layton <jlayton@redhat.com>
Signed-off-by: NSteve French <sfrench@us.ibm.com>

fs/cifs/cifsfs.c

@@ -563,6 +563,10 @@ cifs_get_root(struct smb_vol *vol, struct super_block *sb)
 		mutex_unlock(&dir->i_mutex);
 		dput(dentry);
 		dentry = child;
+		if (!dentry->d_inode) {
+			dput(dentry);
+			dentry = ERR_PTR(-ENOENT);
+		}
 	} while (!IS_ERR(dentry));
 	_FreeXid(xid);
 	kfree(full_path);