9p: don't use __getname/__putname for uname/aname

These are generally very small strings. We don't need an entire 4k
allocation for each. Instead, just free and reallocate them on an
as-needed basis.

Note: This patch is untested since I don't have a 9p server available at
the moment. It's mainly something I noticed while doing some
getname/putname cleanup work.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Eric Van Hensbergen <ericvh@gmail.com>