net/9p: enable 9p to work in non-default network namespace

Teach 9p filesystem to work in container with non-default network namespace.
(Note: I also patched the unix domain socket code but don't have a test case
for that.  It's the same fix, I just don't have a server for it...)

To test, run diod server (http://code.google.com/p/diod):
  diod -n -f -L stderr -l 172.23.255.1:9999 -c /dev/null -e /root
and then mount like so:
  mount -t 9p -o port=9999,aname=/root,version=9p2000.L 172.23.255.1 /mnt

A container test environment is described at http://landley.net/lxc

Signed-off-by: Rob Landley <rob@landley.net>
Signed-off-by: Eric Van Hensbergen <ericvh@gmail.com>