dm-delay: avoid duplicate logic

This is small refactoring of dm-delay - we avoid duplicate logic in
flush_delayed_bios and flush_delayed_bios_fast and join these two
functions into one.

We also add cond_resched() to flush_delayed_bios because the list may have
unbounded number of entries.

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