pci/msi: Retrieve affinity for a vector

Add a helper to get the affinity mask for a given PCI irq vector.  For MSI or
MSI-X vectors these are stored by the IRQ core, while for legacy interrupts
we will always return cpu_possible_map.

[hch: updated to follow the style of pci_irq_vector()]

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Cc: axboe@fb.com
Cc: keith.busch@intel.com
Cc: agordeev@redhat.com
Cc: linux-block@vger.kernel.org
Link: http://lkml.kernel.org/r/1473862739-15032-6-git-send-email-hch@lst.de
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>