zebra: fix installation of LDP labels on static routes

zebra: fix installation of LDP labels on static routes

If the ifindex of a nexthop is not zero, we can't assume that its type is
NEXTHOP_TYPE_IPV4_IFINDEX or NEXTHOP_TYPE_IPV6_IFINDEX. Nexthops of type
NEXTHOP_TYPE_IPV[46] can have their ifindex set by the nexthop_active()
function.

With that said, we need to me more flexible when comparing nexthops
on mpls_ftn_update() to make sure that we'll always find the correct
route/nexthop to update.

Regression introduced by commit 88d88a ("zebra/ldpd: allow MPLS ECMP on
unnumbered interfaces") and found by ANVL.

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>