vfs: Fix invalid ida_remove() call

When the group id of a shared mount is not allocated, the umount still
tries to call mnt_release_group_id(), which eventually hits a kernel
warning at ida_remove() spewing a message like:
  ida_remove called for id=0 which is not allocated.

This patch fixes the bug simply checking the group id in the caller.
Reported-by: Cristian Rodríguez <crrodriguez@opensuse.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/pnode.c

@@ -83,7 +83,8 @@ static int do_make_slave(struct mount *mnt)
 		if (peer_mnt == mnt)
 			peer_mnt = NULL;
 	}
-	if (IS_MNT_SHARED(mnt) && list_empty(&mnt->mnt_share))
+	if (mnt->mnt_group_id && IS_MNT_SHARED(mnt) &&
+	    list_empty(&mnt->mnt_share))
 		mnt_release_group_id(mnt);
 
 	list_del_init(&mnt->mnt_share);