genirq/msi: Free the fwnode created by msi_create_device_irq_domain()

msi_create_device_irq_domain() creates a firmware node for the new domain,
which is never freed. kmemleak reports:

unreferenced object 0xffff888120ba9a00 (size 96):
  comm "systemd-modules", pid 221, jiffies 4294893411 (age 635.732s)
  hex dump (first 32 bytes):
    00 00 00 00 00 00 00 00 e0 19 8b 83 ff ff ff ff  ................
    00 00 00 00 00 00 00 00 18 9a ba 20 81 88 ff ff  ........... ....
  backtrace:
    [<000000008cdbc98d>] __irq_domain_alloc_fwnode+0x51/0x2b0
    [<00000000c57acf9d>] msi_create_device_irq_domain+0x283/0x670
    [<000000009b567982>] __pci_enable_msix_range+0x49e/0xdb0
    [<0000000077cc1445>] pci_alloc_irq_vectors_affinity+0x11f/0x1c0
    [<00000000532e9ef5>] mlx5_irq_table_create+0x24c/0x940 [mlx5_core]
    [<00000000fabd2b80>] mlx5_load+0x1fa/0x680 [mlx5_core]
    [<000000006bb22ae4>] mlx5_init_one+0x485/0x670 [mlx5_core]
    [<00000000eaa5e1ad>] probe_one+0x4c2/0x720 [mlx5_core]
    [<00000000df8efb43>] local_pci_probe+0xd6/0x170
    [<0000000085cb9924>] pci_device_probe+0x231/0x6e0

Use the proper free operation for the firmware wnode so the name is freed
during error unwind of msi_create_device_irq_domain() and also free the
node in msi_remove_device_irq_domain() if it was automatically allocated.

To avoid extra NULL pointer checks make irq_domain_free_fwnode() tolerant
of NULL.

Fixes: 27a6dea3

 ("genirq/msi: Provide msi_create/free_device_irq_domain()")
Reported-by: Omri Barazi <obarazi@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Kalle Valo <kvalo@kernel.org>
Tested-by: Leon Romanovsky <leonro@nvidia.com>
Link: https://lore.kernel.org/r/0-v2-24af6665e2da+c9-msi_leak_jgg@nvidia.com