The info->target comes from userspace and it would be used directly.
So we need to add the sanity check to make sure it is a valid standard
target, although the ebtables tool has already checked it. Kernel needs
to validate anything coming from userspace.

If the target is set as an evil value, it would break the ebtables
and cause a panic. Because the non-standard target is treated as one
offset.

Now add one helper function ebt_invalid_target, and we would replace
the macro INVALID_TARGET later.

Signed-off-by: Gao Feng <gfree.wind@vip.163.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>