do_move_mount(): fix an unsafe use of is_anon_ns()

What triggers it is a race between mount --move and umount -l
of the source; we should reject it (the source is parentless *and*
not the root of anon namespace at that), but the check for namespace
being an anon one is broken in that case - is_anon_ns() needs
ns to be non-NULL.  Better fixed here than in is_anon_ns(), since
the rest of the callers is guaranteed to get a non-NULL argument...

Reported-by: syzbot+494c7ddf66acac0ad747@syzkaller.appspotmail.com
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/namespace.c

@@ -2599,7 +2599,7 @@ static int do_move_mount(struct path *old_path, struct path *new_path)
  if (attached && !check_mnt(old))
  goto out;
 
- if (!attached && !is_anon_ns(ns))
+ if (!attached && !(ns && is_anon_ns(ns)))
  goto out;
 
  if (old->mnt.mnt_flags & MNT_LOCKED)