PCI/MSI: Cleanup msi_mask()

msi_mask() is calculating the possible mask bits for MSI per vector
masking.

Rename it to msi_multi_mask() and hand the MSI descriptor pointer into it
to simplify the call sites.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20210729222543.203905260@linutronix.de