p9fs: fix lookup of "." for lib9p-based 9P servers

The lib9p implementation takes a strict interpretation of the Twalk RPC
call and returns an error for attempts to lookup ".". The workaround is
to fake the lookup locally.

Reviewed by: Val Packett <val@packett.cool>
MFC after: 3 months

(cherry picked from commit 56e4622588ed2eec0197ac47c3059d3db439f5c0)