MFC r270348:
Add some missing checks for unsupported interfaces (e.g. pflog(4)) when
handling ioctls. While here, remove duplicated checks for a NULL ifp in
in6_control(): this check is already done near the beginning of the
function.

MFC r270349:
Suppress warnings when retrieving protocol stats from interfaces that
don't support IPv6 (e.g. pflog(4)).

PR: 189117
Approved by: re (gjb)