On systems with a non power-of-two CPU count the existing MSI-X grouping
code failed to distribute interrupts correctly. Rework the code to
handle arbitrary processor counts.

Also remove the hardcoded upper limit on the number of processors so we
can boot on large systems.
Signed-off-by: NMartin K. Petersen <martin.petersen@oracle.com>
Acked-by: NSreekanth Reddy <Sreekanth.reddy@avagotech.com>
Signed-off-by: NChristoph Hellwig <hch@lst.de>