netfilter: ebtables: Fix extension lookup with identical name

If a requested extension exists as module and is not loaded,
ebt_check_match() might accidentally use an NFPROTO_UNSPEC one with same
name and fail.

Reproduced with limit match: Given xt_limit and ebt_limit both built as
module, the following would fail:

  modprobe xt_limit
  ebtables -I INPUT --limit 1/s -j ACCEPT

The fix is to make ebt_check_match() distrust a found NFPROTO_UNSPEC
extension and retry after requesting an appropriate module.

Cc: Florian Westphal <fw@strlen.de>
Signed-off-by: Phil Sutter <phil@nwl.cc>
Acked-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>