Fill in the export_perm and then pass it to get_req_uid_gid

Manage_Gids option works with NFSv3 but fails with NFSv4. We need to
call nfs_export_check_access() first to fill in the export_perm, and
then call get_req_uid_gid().

Signed-off-by: Philippe DENIEL <philippe.deniel@cea.fr>