dm/core: Rename kcopyd_job.rw into kcopyd.op

The member name 'rw' suggests that this member either has the value 'READ'
or 'WRITE' and no other values. Since that member also can have the value
REQ_OP_WRITE_ZEROES, rename 'rw' into 'op'. This patch does not change any
functionality since REQ_OP_READ = READ = 0 and REQ_OP_WRITE = WRITE = 1.

Cc: Alasdair Kergon <agk@redhat.com>
Cc: Mike Snitzer <snitzer@kernel.org>
Cc: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Link: https://lore.kernel.org/r/20220714180729.1065367-23-bvanassche@acm.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>