vfio/pci: Make VFIO_PCI_OFFSET_TO_INDEX() return unsigned

vfio/pci: Make VFIO_PCI_OFFSET_TO_INDEX() return unsigned

VFIO_PCI_OFFSET_TO_INDEX() is used in several places with a signed
parameter (e.g. loff_t).  Because it makes no sense for a BAR/resource
index to be negative, enforce this in the macro.

This fixes at least one current issue, where vfio_pci_ioeventfd() uses
this macro with an unvalidated signed loff_t returned into a signed
type, leading to a possible negative array access.  This instance does
test against an out-of-bounds positive value, so treating the index as
unsigned fixes this issue.

Fixes: 89e1f7d4c66d8 ("vfio: Add PCI device driver")
Signed-off-by: Matt Evans <mattev@meta.com>
Link: https://lore.kernel.org/r/20260511144642.2926799-1-mattev@meta.com
Signed-off-by: Alex Williamson <alex@shazbot.org>