target: Move cmd->prot_op check into target_read_prot_action

This patch moves the existing target_complete_ok_work() check for
cmd->prot_op into it's own function, so it's easier to add future
support for READ INSERT.

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>