dm-verity: consolidate the BH and normal work structs

dm-verity: consolidate the BH and normal work structs

Since each dm_verity_io is never on both the BH and normal workqueues at
the same time, there's no need for two different work_structs.  Replace
the 'bh_work' and 'work' fields with just 'work'.

Note: this is correct even though it means 'work' may be reused while
verity_bh_work() is running.  The workqueue API allows work functions to
reuse or free their work_struct, and many workqueue users rely on that.

Signed-off-by: Eric Biggers <ebiggers@kernel.org>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>