dm-verity: fix up various workqueue-related comments

dm-verity: fix up various workqueue-related comments

Replace obsolete mentions of "tasklets" with "softirq context", and
"workqueue" with "kworker".

This reflects the fact that the implementation of the
"try_verify_in_tasklet" dm-verity option now accesses softirq context
using either the BH workqueue API or inline execution, not the tasklet
API.  The old names conflated the API with the intended execution
context, so they became outdated when the APIs changed.

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