vfs: fix nd->root leak in do_filp_open()

commit 2a737871 "Cache root in nameidata"
introduced a new member nd->root, but forgot to put it in do_filp_open().

Signed-off-by: J. R. Okajima <hooanon05@yahoo.co.jp>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>