Fix "netstat -gW" behavior broken in r259638.

Fix "netstat -gW" behavior broken in r259638.
netstat has two options for printing multicast tables:
sysctl (the default one for live systems) and kvm-based one (for cores).
It looks like kvm-based one hasn't been working since it's been introduced
in r190012 due to absence of mfctablesize kernel symbol.
Check for all ipv4-multicast symbols being correctly resolved was introduced
in r259638 regardless of 'live' value leading to "No IPv4 MROUTING" error
message.

Reported by:	Olivier Cochard-Labbé
MFC after:	1 week