PCI/MSI: Export all remapped MSIs to sysfs attributes

irqbalance uses sysfs attributes to populate its internal database, which
is then used to bind the IRQ to the appropriate NUMA node.

On a device accepting multiple MSIs and with interrupt remapping enabled,
only the first IRQ entry is exported in the "msi_irqs" directory.  This
results in irqbalance having no clue of the NUMA affinity for the extra
IRQs, so it can't bind them to the correct node.

Export all MSI interrupts as sysfs attributes when relevant.

[bhelgaas: changelog]
Signed-off-by: Romain Bezut <rbezut@gmail.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Thomas Gleixner <tglx@linutronix.de>