take freeing of emptied mnt_namespace to namespace_unlock()

take freeing of emptied mnt_namespace to namespace_unlock()

Freeing of a namespace must be delayed until after we'd dealt with mount
notifications (in namespace_unlock()).  The reasons are not immediately
obvious (they are buried in ->prev_ns handling in mnt_notify()), and
having that free_mnt_ns() explicitly called after namespace_unlock()
is asking for trouble - it does feel like they should be OK to free
as soon as they've been emptied.

Make the things more explicit by setting 'emptied_ns' under namespace_sem
and having namespace_unlock() free the sucker as soon as it's safe to free.

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