p9fs: Properly handle paths containing "."

Fixes: 56e4622588ed ("p9fs: fix lookup of "." for lib9p-based 9P servers")
Pull Request: https://github.com/freebsd/freebsd-src/pull/1361

(cherry picked from commit 62bb18abed25a831a29e1e5b6e6f7fdddbba6403)