dm: use signal_pending_state() in dm_wait_for_completion()

Use signal_pending_state() instead of open-coding it.  This patch does
not change any functionality but makes it possible to pass TASK_KILLABLE
as the second argument of dm_wait_for_completion().  See also commit
16882c1e ("sched: fix TASK_WAKEKILL vs SIGKILL race").

Signed-off-by: Bart Van Assche <bart.vanassche@sandisk.com&gt;.>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>