ovl: Fix OVL_XATTR_PREFIX

Make sure ovl_own_xattr_handler only matches attribute names starting
with "overlay.", not "overlayXXX".

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Fixes: d837a49b ("ovl: fix POSIX ACL setting")
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>