This allows drivers to retrieve the Linux interrupt number instead of
fiddling with MSI descriptors.

msi_get_virq() returns the Linux interrupt number or 0 in case that there
is no entry for the given MSI index.
Signed-off-by: NThomas Gleixner <tglx@linutronix.de>
Tested-by: NMichael Kelley <mikelley@microsoft.com>
Tested-by: NNishanth Menon <nm@ti.com>
Reviewed-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: NJason Gunthorpe <jgg@nvidia.com>
Link: https://lore.kernel.org/r/20211210221814.780824745@linutronix.de