target: don't depend on SCSI

The core target code only needs code from scsi_common.c, which is now
separately selectable.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>