iscsi-target: fix iscsi cmd leak

If iscsi-target receives NOP OUT with ITT and TTT
set to 0xffffffff it allocates iscsi_cmd but
does not free the cmd, so free iscsi_cmd in this case.

Signed-off-by: Varun Prakash <varun@chelsio.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>