scsi: snic: Fix SCSI host leak on workqueue allocation failure

In snic_add_host(), if scsi_add_host() succeeds but
alloc_ordered_workqueue() fails, the function returns -ENOMEM with
shost->work_q left as NULL. The caller's error path then calls
snic_del_host(), which returns early when !shost->work_q without calling
scsi_remove_host(). The Scsi_Host remains registered in sysfs as a zombie
device even after the probe has failed. This causes:

 - The leaked host remains visible in /sys/class/scsi_host/ after probe
   failure, with state "running".

 - Subsequent SCSI host numbering is permanently shifted (the leaked host
   ID from ida_alloc() is never reclaimed).

 - Memory leak: the Scsi_Host allocation can never be freed because
   device_add() took a reference that can only be released by device_del()
   inside scsi_remove_host().

Fix by adding scsi_remove_host() in the workqueue allocation failure path
inside snic_add_host(), undoing the successful scsi_add_host() before
returning the error. This is cleaner than modifying snic_del_host() because
snic_del_host() is called from a shared error label that also serves paths
where snic_add_host() was never invoked.

Reproducer (requires no real SNIC hardware):

 - Build CONFIG_SCSI_SNIC=y (built-in)

 - Add snic.test_mode=1 snic.inject_wq_fail=1 to kernel cmdline

 - Boot with a PCI device matching the snic driver (e.g. QEMU edu device,
   PCI ID 0x1234:0x11e8, temporarily added to the driver's PCI ID table)

Before the fix:

  # /sys/class/scsi_host/ contains a zombie host0:
  $ cat /sys/class/scsi_host/host0/proc_name
  snic_scsi
  $ cat /sys/class/scsi_host/host0/state
  running
  # ata_piix gets host1, host2 (host0 stuck):
  scsi host1: ata_piix
  scsi host2: ata_piix

After the fix:

  # host0 is properly freed and reused by ata_piix:
  scsi host0: ata_piix
  scsi host1: ata_piix
  # No zombie host in /sys/class/scsi_host/

Signed-off-by: Chen Changcheng <chenchangcheng@kylinos.cn>
Acked-by: Narsimhulu Musini <nmusini@cisco.com>
Link: https://patch.msgid.link/20260727073438.209673-1-chenchangcheng@kylinos.cn
Signed-off-by: Martin K. Petersen (Oracle) <mkp@kernel.org>

drivers/scsi/snic/snic_main.c

@@ -305,6 +305,7 @@ snic_add_host(struct Scsi_Host *shost, struct pci_dev *pdev)
 	if (!shost->work_q) {
 		SNIC_HOST_ERR(shost, "Failed to Create ScsiHost wq.\n");
 
+		scsi_remove_host(shost);
 		ret = -ENOMEM;
 	}