iscsi-target: Fix CmdSN comparison (use cmd->cmd_sn instead of cmd->stat_sn)

Commit 64c13330a389 ("iscsi-target: Fix bug in handling of ExpStatSN
ACK during u32 wrap-around") introduced a bug where we compare the
wrong SN against our ExpCmdSN.

Reported-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Roland Dreier <roland@purestorage.com>
Cc: stable@vger.kernel.org
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>