PCI/MSI: Skip MSI/MSI-X programming while the channel is offline

The MSI-X Table lives in device MMIO space behind a BAR and the MSI Mask
register in Configuration Space, so neither is reachable while the Link is
down. While a Downstream Port has the Link contained by DPC it completes
these accesses with Unsupported Request, and reads return all ones.

If the upstream Root Port implements the RP Extensions for DPC, it
additionally reports that UR completion as an RP PIO error and triggers a
second containment event, this time at the Root Port, which contains every
device below it. So a contained Link on one Downstream Port turns into a
far wider outage that takes down unrelated devices.

pci_free_irq_vectors() is called from driver error_detected() and
prepare-for-reset callbacks, i.e. while the Link is contained and before
the reset and the pci_restore_state() that follows it, and it masks every
descriptor. Skip the programming when pci_channel_offline(), which also
covers surprise removal. The msix_ctrl and msi_mask caches are still
updated, so the restore paths replay the intended state once the Link is
back up, and report_slot_reset() clears the offline state before the driver
callback runs, so recovery is unaffected.  pci_msix_write_tph_tag() flushes
its Vector Control update with an unconditional read, so return -EIO there
rather than issue it for a write that was skipped; the caller disables TPH
in response. error_state is only set once containment has occurred, so this
covers the case where the kernel knows the Link is down; it is not mutual
exclusion against a containment event that begins concurrently.

This does not attempt to make every Configuration Space access safe while
the Link is contained.

Signed-off-by: Vidya Sagar <vidyas@nvidia.com>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Link: https://patch.msgid.link/20260909164908.2562818-1-vidyas@nvidia.com

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