lockdep: check that no locks held at freeze time

lockdep: check that no locks held at freeze time

We shouldn't try_to_freeze if locks are held.  Verified that I get no
lockdep warnings after applying this patch and "vfork: don't
freezer_count() for in-kernel users of CLONE_VFORK".

Signed-off-by: Mandeep Singh Baines <msb@chromium.org>
Cc: Ben Chan <benchan@chromium.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Rafael J. Wysocki <rjw@sisk.pl>
Cc: Ingo Molnar <mingo@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>