iscsi: recognize "invalid field" ASCQ from WRITE SAME command

Some targets may return "invalid field" as the ASCQ from WRITE SAME
if they support the command only without the UNMAP field.  Recognize
that, and return ENOTSUP just like for "invalid operation code".

Reviewed-by: Peter Lieven <pl@kamp.de>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
