zebra: optimization on the mac addition for evpn-mh

zebra: optimization on the mac addition for evpn-mh

When `zebra_evpn_mac_svi_add()` adds one found mac by
`zebra_evpn_mac_lookup()` and the found mac is without
svi flag, then call `zebra_evpn_mac_svi_add()` to create
one appropriate mac, but it will call `zebra_evpn_mac_lookup()`
the second time. So lookup twice, the procedure is redundant.

Just an optimization for it, make sure only lookup once.

Modify `zebra_evpn_mac_gw_macip_add()` to check the `macp`
parameter passed by caller, so it can distinguish whether
really need lookup or not.

Signed-off-by: anlan_cs <vic.lan@pica8.com>