xen-netfront: release grant references used for the shared rings

Just calling gnttab_end_foreign_access_ref doesn't free the references,
instead call gnttab_end_foreign_access with a NULL page argument in order to
have the grant references freed. The code that maps the ring
(xenbus_map_ring) already uses gnttab_grant_foreign_access which takes care
of allocating a grant reference.

Reviewed by: Wei Liu <wei.liu2@citrix.com>
Sponsored by: Citrix Systems R&D
Differential revision: https://reviews.freebsd.org/D6608