dm io: remove code duplication between sync_io and aysnc_io

The only difference between the code to setup and dispatch the io in
sync_io() and async_io() is the sync argument to dispatch_io(), which
is used to update the opf argument.

Update the opf argument direcly in sync_io(), and remove the sync
argument from dispatch_io(). Then, make sync_io() call async_io()
instead of duplicting all of its code.

Signed-off-by: Benjamin Marzinski <bmarzins@redhat.com>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>