lib/zebra: put vrf_get() on a diet

lib/zebra: put vrf_get() on a diet

Also, for some reason we had two functions to search a VRF by its name:
zebra_vrf_lookup_by_name() and zebra_vrf_list_lookup_by_name().  The first
one would loop through vrf_table and the other one through vrf_list. This
is not necessary anymore, so remove zebra_vrf_lookup_by_name() and rename
zebra_vrf_list_lookup_by_name() to zebra_vrf_lookup_by_name().

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