sockstat: Surround explicit IPv6 addresses with brackets

Approved by: re (cperciva)
PR: 254611
Approved by: otis, tuexen, des
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D54375

(cherry picked from commit fe81e3944c085e765c83c4f78941d7529ceb556e)
(cherry picked from commit 03c8e3e77114530f61e0dd9f2b1757eeea033d0e)