iscsi-target: Do not generate REJECTs for zero-length DataOUT

This patch changes iscsit_check_dataout_hdr() to no longer generate
REJECTs for zero-length DataOUTs, and instead simply ignore these
requests.

This follows RFC-3720, Section 10.7.7.  DataSegmentLength

  "This is the data payload length of a SCSI Data-In or SCSI Data-Out PDU.
   The sending of 0 length data segments should be avoided, but initiators
   and targets MUST be able to properly receive 0 length data segments."

Reported-by: Santosh Kulkarni <santosh.kulkarni@calsoftinc.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>