target/iscsi: Remove second argument of __iscsit_free_cmd()

Initialize .data_direction to DMA_NONE in iscsit_allocate_cmd()
such that the second argument of __iscsit_free_cmd() can be left
out. Note: this patch causes the first part of __iscsit_free_cmd()
no longer to be skipped for TMFs. That's fine since no data
segments are associated with TMFs.

Signed-off-by: Bart Van Assche <bart.vanassche@sandisk.com>
Reviewed-by: Hannes Reinecke <hare@suse.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Andy Grover <agrover@redhat.com>
Cc: David Disseldorp <ddiss@suse.de>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>