target: Move cmd->prot_op check into target_write_prot_action
This patch moves the existing target_execute_cmd() check for
cmd->prot_op into it's own function, so it's easier to add
future support for WRITE STRIP.
(Use better target_write_prot_action name - Sagi)
Reviewed-by: Martin Petersen <martin.petersen@oracle.com> Reviewed-by: Sagi Grimberg <sagig@mellanox.com> Cc: Christoph Hellwig <hch@lst.de> Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>