PCI/MSI: Enforce that MSI-X table entry is masked for update

The specification (PCIe r5.0, sec 6.1.4.5) states:

    For MSI-X, a function is permitted to cache Address and Data values
    from unmasked MSI-X Table entries. However, anytime software unmasks a
    currently masked MSI-X Table entry either by clearing its Mask bit or
    by clearing the Function Mask bit, the function must update any Address
    or Data values that it cached from that entry. If software changes the
    Address or Data value of an entry while the entry is unmasked, the
    result is undefined.

The Linux kernel's MSI-X support never enforced that the entry is masked
before the entry is modified hence the Fixes tag refers to a commit in:
      git://git.kernel.org/pub/scm/linux/kernel/git/tglx/history.git



Enforce the entry to be masked across the update.

There is no point in enforcing this to be handled at all possible call
sites as this is just pointless code duplication and the common update
function is the obvious place to enforce this.

Fixes: f036d4ea5fa7 ("[PATCH] ia32 Message Signalled Interrupt support")
Reported-by: Kevin Tian <kevin.tian@intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Marc Zyngier <maz@kernel.org>
Acked-by: Bjorn Helgaas <bhelgaas@google.com>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20210729222542.462096385@linutronix.de