netfilter: bridge: build br_nf_core only if required

Eric reports build failure with
CONFIG_BRIDGE_NETFILTER=n

We insist to build br_nf_core.o unconditionally, but we must only do so
if br_netfilter was enabled, else it fails to build due to
functions being defined to empty stubs (and some structure members
being defined out).

Also, BRIDGE_NETFILTER=y|m makes no sense when BRIDGE=n.

Fixes: 34666d46 (netfilter: bridge: move br_netfilter out of the core)
Reported-by: NEric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: NFlorian Westphal <fw@strlen.de>
Signed-off-by: NPablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/Kconfig

@@ -177,7 +177,7 @@ config NETFILTER_ADVANCED
 
 config BRIDGE_NETFILTER
 	tristate "Bridged IP/ARP packets filtering"
-	depends on (BRIDGE || BRIDGE=n)
+	depends on BRIDGE
 	depends on NETFILTER && INET
 	depends on NETFILTER_ADVANCED
 	default m

net/bridge/Makefile

@@ -6,11 +6,12 @@ obj-$(CONFIG_BRIDGE) += bridge.o
 
 bridge-y	:= br.o br_device.o br_fdb.o br_forward.o br_if.o br_input.o \
 			br_ioctl.o br_stp.o br_stp_bpdu.o \
-			br_stp_if.o br_stp_timer.o br_netlink.o \
-			br_nf_core.o
+			br_stp_if.o br_stp_timer.o br_netlink.o
 
 bridge-$(CONFIG_SYSFS) += br_sysfs_if.o br_sysfs_br.o
 
+bridge-$(subst m,y,$(CONFIG_BRIDGE_NETFILTER)) += br_nf_core.o
+
 obj-$(CONFIG_BRIDGE_NETFILTER) += br_netfilter.o
 
 bridge-$(CONFIG_BRIDGE_IGMP_SNOOPING) += br_multicast.o br_mdb.o