dm delay: refactor repetitive code

dm-delay has a lot of code that is repeated for delaying read and write
bios.  Repetitive code is generally bad; refactor out the repetitive
code in preperation for adding another delay class for flush bios.

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