Fix style bugs in in6_pcblookup_lbgroup().

This should have been a part of r338470. No functional changes
intended.

Reported by: gallatin
Reviewed by: gallatin, Johannes Lundberg <johalun0@gmail.com>
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D17109