vfio/pci: Check BAR resources before exporting a DMABUF

vfio/pci: Check BAR resources before exporting a DMABUF

A DMABUF exports access to BAR resources and, although they are
requested at startup time, we need to ensure they really were reserved
before exporting.  Otherwise, it's possible to access unreserved
resources through the export.

Add a check to the DMABUF-creation path.

Fixes: 5d74781ebc86c ("vfio/pci: Add dma-buf export support for MMIO regions")
Signed-off-by: Matt Evans <mattev@meta.com>
Link: https://lore.kernel.org/r/20260511145829.2993601-3-mattev@meta.com
Signed-off-by: Alex Williamson <alex@shazbot.org>