iscsi-target: Add missing CMDSN_LOWER_THAN_EXP check in iscsit_handle_scsi_cmd

This patch adds a missing CMDSN_LOWER_THAN_EXP return check for
iscsit_sequence_cmd() in iscsit_handle_scsi_cmd() that was incorrectly
dropped during the v3.1-rc cleanups to use iscsit_sequence_cmd().

Cc: Andy Grover <agrover@redhat.com>
Cc: stable@kernel.org
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>