dm crypt: use wake_up_process() instead of a wait queue

This is a small simplification of dm-crypt - use wake_up_process()
instead of a wait queue in a case where only one process may be
waiting.  dm-writecache uses a similar pattern.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>