NTB: epf: Avoid calling pci_irq_vector() from hardirq context

NTB: epf: Avoid calling pci_irq_vector() from hardirq context

ntb_epf_vec_isr() calls pci_irq_vector() in hardirq context to derive
the vector number. pci_irq_vector() calls msi_get_virq() that takes a
mutex and can therefore trigger "scheduling while atomic" splats:

  BUG: scheduling while atomic: kworker/u33:0/55/0x00010001
  ...
  Call trace:
   ...
   schedule+0x38/0x110
   schedule_preempt_disabled+0x28/0x50
   __mutex_lock.constprop.0+0x848/0x908
   __mutex_lock_slowpath+0x18/0x30
   mutex_lock+0x4c/0x60
   msi_domain_get_virq+0xe8/0x138
   pci_irq_vector+0x2c/0x60
   ntb_epf_vec_isr+0x28/0x120 [ntb_hw_epf]
   __handle_irq_event_percpu+0x70/0x3a8
   handle_irq_event+0x48/0x100
   handle_edge_irq+0x100/0x1c8
   ...

Cache the Linux IRQ number for vector 0 when vectors are allocated and
use it as a base in the ISR. Running the ISR in a threaded IRQ handler
would also avoid the problem, but that would be unnecessary here.

Fixes: 812ce2f8d14e ("NTB: Add support for EPF PCI Non-Transparent Bridge")
Signed-off-by: Koichiro Den <den@valinux.co.jp>
Signed-off-by: Manivannan Sadhasivam <mani@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Cc: stable@vger.kernel.org # v5.12+
Link: https://patch.msgid.link/20260304083028.1391068-3-den@valinux.co.jp