netfilter: physdev: relax br_netfilter dependency

Following command:
  iptables -D FORWARD -m physdev ...
causes connectivity loss in some setups.

Reason is that iptables userspace will probe kernel for the module revision
of the physdev patch, and physdev has an artificial dependency on
br_netfilter (xt_physdev use makes no sense unless a br_netfilter module
is loaded).

This causes the "phydev" module to be loaded, which in turn enables the
"call-iptables" infrastructure.

bridged packets might then get dropped by the iptables ruleset.

The better fix would be to change the "call-iptables" defaults to 0 and
enforce explicit setting to 1, but that breaks backwards compatibility.

This does the next best thing: add a request_module call to checkentry.
This was a stray '-D ... -m physdev' won't activate br_netfilter
anymore.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

include/net/netfilter/br_netfilter.h

@@ -43,7 +43,6 @@ static inline struct rtable *bridge_parent_rtable(const struct net_device *dev)
 }
 
 struct net_device *setup_pre_routing(struct sk_buff *skb);
-void br_netfilter_enable(void);
 
 #if IS_ENABLED(CONFIG_IPV6)
 int br_validate_ipv6(struct net *net, struct sk_buff *skb);