iscsi-target: Correctly set 0xffffffff field within ISCSI_OP_REJECT PDU

This patch adds a missing iscsi_reject->ffffffff assignment within
iscsit_send_reject() code to properly follow RFC-3720 Section 10.17
Bytes 16 -> 19 for the PDU format definition of ISCSI_OP_REJECT.

We've not seen any initiators care about this bytes in practice, but
as Ronnie reported this was causing trouble with wireshark packet
decoding lets go ahead and fix this up now.

Reported-by: Ronnie Sahlberg <ronniesahlberg@gmail.com>
Cc: stable@vger.kernel.org
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>