9pfs: local: link: don't follow symlinks

The local_link() callback is vulnerable to symlink attacks because it calls:

(1) link() which follows symbolic links for all path elements but the
    rightmost one
(2) local_create_mapped_attr_dir()->mkdir() which follows symbolic links
    for all path elements but the rightmost one

This patch converts local_link() to rely on opendir_nofollow() and linkat()
to fix (1), mkdirat() to fix (2).

This partly fixes CVE-2016-9602.

Signed-off-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
