block: Fix secure erase

Commit 288dab8a

 ("block: add a separate operation type for secure
erase") split REQ_OP_SECURE_ERASE from REQ_OP_DISCARD without considering
all the places REQ_OP_DISCARD was being used to mean either. Fix those.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Fixes: 288dab8a

 ("block: add a separate operation type for secure erase")
Signed-off-by: Jens Axboe <axboe@fb.com>