lio_ha: add TrueNAS LIO HA forwarding module

Introduces lio_ha.ko, a kernel module that implements active/standby
high-availability for the LIO SCSI target subsystem on TrueNAS.

On the STANDBY node, the module intercepts every SCSI command, TMR,
and PR OUT before it reaches the local backend and forwards it to the
ACTIVE node over a private TCP channel (port 999).  On failover a
single write to a configfs attribute clears the forwarding flag;
lio_ha restores replicated Persistent Reservation state directly into
LIO core, opens the local iblock/fileio backends, and resumes local
execution -- without removing or reinserting any LUN.

On the ACTIVE node, a private fabric driver (ha_recv) accepts the TCP
connection from STANDBY, creates a synthetic se_session for each
initiator with the correct node_acl, and submits forwarded commands to
LIO core for local execution under the right I_T nexus context so that
PR per-I_T nexus checking remains correct across the HA pair.

The module is loaded on both nodes.  A single persistent TCP
connection carries session lifecycle, I/O, TMR, PR replication
(PERS_ACTION), and bulk PR sync (LUN_SYNC) traffic.

Enabled by CONFIG_LIO_HA (tristate, depends on CONFIG_TRUENAS).

Author: bmeagherix

drivers/target/Kconfig

@@ -48,5 +48,6 @@ source "drivers/target/tcm_fc/Kconfig"
 source "drivers/target/iscsi/Kconfig"
 source "drivers/target/sbp/Kconfig"
 source "drivers/target/tcm_remote/Kconfig"
+source "drivers/target/lio_ha/Kconfig"
 
 endif

drivers/target/Makefile

@@ -31,3 +31,4 @@ obj-$(CONFIG_TCM_FC)		+= tcm_fc/
 obj-$(CONFIG_ISCSI_TARGET)	+= iscsi/
 obj-$(CONFIG_SBP_TARGET)	+= sbp/
 obj-$(CONFIG_REMOTE_TARGET)	+= tcm_remote/
+obj-$(CONFIG_LIO_HA)		+= lio_ha/

drivers/target/lio_ha/Kconfig

@@ -0,0 +1,22 @@
+# SPDX-License-Identifier: GPL-2.0-only
+
+config LIO_HA
+	tristate "TrueNAS LIO HA forwarding module"
+	depends on TARGET_CORE && TRUENAS
+	help
+	  Enables the lio_ha.ko kernel module for TrueNAS High Availability
+	  with the Linux LIO iSCSI/FC target stack.
+
+	  On a STANDBY node, lio_ha intercepts all I/O and Task Management
+	  Requests before they reach the local backend and forwards them to
+	  the ACTIVE node over a private TCP channel.  On failover, a single
+	  write clears the forwarding flag; lio_ha restores replicated
+	  Persistent Reservation state, opens the local iblock/fileio backends,
+	  and stops forwarding -- without any LUN removal or reinsertion.
+
+	  The module is loaded on both ACTIVE and STANDBY nodes.  On ACTIVE
+	  it registers the ha_recv fabric driver that accepts forwarded commands
+	  and submits them to the local LIO stack on behalf of STANDBY's
+	  initiators, preserving correct per-I_T nexus PR checking.
+
+	  If unsure, say N.

drivers/target/lio_ha/Makefile

@@ -0,0 +1,5 @@
+# SPDX-License-Identifier: GPL-2.0-only
+
+lio_ha-y := lio_ha_main.o lio_ha_recv.o lio_ha_tcp.o lio_ha_fwd.o
+
+obj-$(CONFIG_LIO_HA) += lio_ha.o