target: replace se_cmd->execute_rw with a protocol_data field

Instead of leaking this SBC read/write implementation detail just add an
opaqueue protocol specific pointer to struct se_cmd that we can assign
the sbc_ops vector to.

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