[SCSI] libiscsi: rename iscsi_cmd_task to iscsi_task

This is the second part of the iscsi task merging, and
all it does it rename iscsi_cmd_task to iscsi_task and
mtask/ctask to just task.

Signed-off-by: Mike Christie <michaelc@cs.wisc.edu>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>