vfio/type1: Use consistent types for page counts

Page count should more consistently be an unsigned long when passed as
an argument while functions returning a number of pages should use a
signed long to allow for -errno.

vaddr_get_pfns() can therefore be upgraded to return long, though in
practice it's currently limited by the batch capacity.  In fact, the
batch indexes are noted to never hold negative values, so while it
doesn't make sense to bloat the structure with unsigned longs in this
case, it does make sense to specify these as unsigned.

No change in behavior expected.

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/20250218222209.1382449-5-alex.williamson@redhat.com


Signed-off-by: Alex Williamson <alex.williamson@redhat.com>