rtnl_route: decode struct rta_mfc_stats and struct rtvia unconditionally

rtnl_route: decode struct rta_mfc_stats and struct rtvia unconditionally

* rtnl_route.c (struct_rta_mfc_stats, struct_rtvia): New typedefs.
[HAVE_STRUCT_RTA_MFC_STATS]: Add a static_assert to check
that sizeof(struct rta_mfc_stats) has the expected value.
[HAVE_STRUCT_RTVIA]: Add a static_assert to check that sizeof(struct
rtvia) has the expected value.
(decode_rta_mfc_stats) [HAVE_STRUCT_RTA_MFC_STATS]: Remove guard.
(decode_rta_mfc_stats): Change the type of mfcs variable to
struct_rta_mfc_stats.
(decode_rtvia): Change the type of via variable to struct_rtvia.

References: https://bugzilla.redhat.com/show_bug.cgi?id=1758201