mount: restore cwd in open_mountpoint()

setns(fd, CLONE_NEWNS) resets cwd and root, so we need to
restore them back.

Without this patch stats-dump isn't saved in the work dir:
-rw-r--r-- 1 root root 32 Apr  2 14:21 /stats-dump

Signed-off-by: Andrey Vagin <avagin@openvz.org>
Acked-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>