sockstat: fix column length for PROTO

The computation of the length was not taking into account that IPv6
endpoints, which are not IPv6 only, have a suffix of 46.
For UDP and TCP this bug was not relevant, since tcp46 and udp46
has the same length as PROTO, but sctp46 is longer. Upcoming
udplite support will also be affected.

Reviewed by: asomers
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D53212