target: simplify UNMAP handling

Move a little more processing into the core code, and lift the previous
do_unmap callback into the sbc_ops structure.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Andy Grover <agrover@redhat.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>