9p: fix bug in attach-per-user

When a new user attached at a directory other than the root, he would end
up in the parent directory of the cwd.  This was due to a logic error in
the code which attaches the user at the mount point and walks back to the
cwd.  This patch fixes that.

Signed-off-by: Eric Van Hensbergen <ericvh@gmail.com>