Don't bother with check_mnt() in do_add_mount() on shrinkable ones

These guys are what we add as submounts; checks for "is that attached in
our namespace" are simply irrelevant for those and counterproductive for
use of private vfsmount trees a-la what NFS folks want.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>