dm/ebs: Change 'int rw' into 'enum req_op op'

Improve static type checking by using type 'enum req_op' instead of 'int'.
Make the role of the 'rw' arguments more clear by renaming these into
'op' (operation). 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: Heinz Mauelshagen <heinzm@redhat.com>
Cc: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Link: https://lore.kernel.org/r/20220714180729.1065367-25-bvanassche@acm.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>