zebra/lib: remove redundant fields from zebra_vrf

zebra/lib: remove redundant fields from zebra_vrf

There's no need to duplicate the 'vrf_id' and 'name' fields from the 'vrf'
structure into the 'zebra_vrf' structure. Instead of that, add a back
pointer in 'zebra_vrf' that should point to the associated 'vrf' structure.

Additionally, modify the vrf callbacks to pass the whole vrf structure
as a parameter. This allow us to make further simplifications in the code.

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