lib: Do not convert EVPN prefixes into IPv4/IPv6 if not needed

lib: Do not convert EVPN prefixes into IPv4/IPv6 if not needed

Convert only when this is really needed, e.g. `match ip address prefix-list ...`.

Otherwise, we can't have mixed match clauses, like:

```
match ip address prefix-list p1
match evpn route-type prefix
```

This won't work, because the prefix is already converted, and we can't extract
route type, vni, etc. from the original EVPN prefix.

Signed-off-by: Donatas Abraitis <donatas@opensourcerouting.org>