iscsi-target: Fix bug in handling of ExpStatSN ACK during u32 wrap-around

This patch fixes a bug in the hanlding of initiator provided ExpStatSN and
individual iscsi_cmd->stat_sn comparision during iscsi_conn->stat_sn
wrap-around within iscsit_ack_from_expstatsn() code.

This bug would manifest itself as iscsi_cmd descriptors not being Acked
by a lower ExpStatSn, causing them to be leaked until an iSCSI connection
or session reinstatement event occurs to release all commands.

Also fix up two other uses of incorrect CmdSN SNA comparison to use wrapper
usage from include/scsi/iscsi_proto.h.

Signed-off-by: Steve Hodgson <steve@purestorage.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>