vfio/pci: Rename PBA offset in device descriptor to fd_offset

The MSI-X capability defines a PBA offset, which is the offset of the PBA
array in the BAR that holds the array.

kvmtool uses the field "pba_offset" in struct msix_cap (which represents
the MSIX capability) to refer to the [PBA offset:BAR] field of the
capability; and the field "offset" in the struct vfio_pci_msix_pba to refer
to offset of the PBA array in the device descriptor created by the VFIO
driver.

As we're getting ready to add yet another field that represents an offset
to struct vfio_pci_msix_pba, try to avoid ambiguities by renaming the
struct's "offset" field to "fd_offset".

No functional change intended.

Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>
Link: https://lore.kernel.org/r/20211012132510.42134-5-alexandru.elisei@arm.com


Signed-off-by: Will Deacon <will@kernel.org>