dm rq: cope with DM device destruction while in dm_old_request_fn()

Fixes a crash in dm_table_find_target() due to a NULL struct dm_table
being passed from dm_old_request_fn() that races with DM device
destruction.

Reported-by:  <artem@flashgrid.io>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Cc: stable@vger.kernel.org