vfio/type1: Catch zero from pin_user_pages_remote()

vfio/type1: Catch zero from pin_user_pages_remote()

pin_user_pages_remote() can currently return zero for invalid args
or zero nr_pages, neither of which should ever happen.  However
vaddr_get_pfns() indicates it should only ever return a positive
value or -errno and there's a theoretical case where this can slip
through and be unhandled by callers.  Therefore convert zero to
-EFAULT.

Reviewed-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Mitchell Augustin <mitchell.augustin@canonical.com>
Tested-by: Mitchell Augustin <mitchell.augustin@canonical.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Link: https://lore.kernel.org/r/20250218222209.1382449-2-alex.williamson@redhat.com
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>