vfs: fix invalid ida_remove() call

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.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Reported-by: Cristian Rodríguez <crrodriguez@opensuse.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>