scsi: move unmap error checking to the complete callback

scsi: move unmap error checking to the complete callback

This will help to account the operation in the following commit.

The difference is that we don't call scsi_disk_req_check_error() before
the 1st discard iteration anymore. That function also checks if
the request is cancelled, however it shouldn't get canceled until it
yields in blk_aio() functions anyway.
Same approach is already used for emulate_write_same.

Signed-off-by: Anton Nefedov <anton.nefedov@virtuozzo.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>
Message-id: 20190923121737.83281-7-anton.nefedov@virtuozzo.com
Signed-off-by: Max Reitz <mreitz@redhat.com>