[SCSI] libiscsi: don't call into lld to cleanup task

In the normal IO path we should not be calling back
into the LLD since the LLD will have cleaned up the
task before or after calling complete pdu.

For the fail_command path we still need to do this
to force the cleanup.

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