scsi-disk: introduce scsi_disk_req_check_error

Commonize all the checks for canceled requests and errors.  The next patch
will add another case to check for, in order to handle passthrough commands.

There is no semantic change here; the only nontrivial modification is in
scsi_write_do_fua, where cancellation has been checked earlier by both
callers.  Thus, the check is replaced with an assertion.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
