zebra: let /32 host route with same IP cross VRF

zebra: let /32 host route with same IP cross VRF

Contraints of host routes are too strict in current code:
Host routes with same destination address and nexthop address are forbidden
even when cross VRFs.

Currently host routes with different destination and nexthop address can cross
VRFs, it is ok. But host routes with same addresses are forbidden to cross VRFs,
it is wrong.

Since different VRFs can have the same addresses, leak specific host route with
the same nexthop address ( it means destination address is same to nexthop
address ) to other VRFs is a normal case.

This commit relaxes that contraints. Host routes with same destination address
and nexthop address are forbidden only when not cross VRFs.

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