netfilter: log: split family specific code to nf_log_{ip,ip6,common}.c files

The plain text logging is currently embedded into the xt_LOG target.
In order to be able to use the plain text logging from nft_log, as a
first step, this patch moves the family specific code to the following
files and Kconfig symbols:

1) net/ipv4/netfilter/nf_log_ip.c: CONFIG_NF_LOG_IPV4
2) net/ipv6/netfilter/nf_log_ip6.c: CONFIG_NF_LOG_IPV6
3) net/netfilter/nf_log_common.c: CONFIG_NF_LOG_COMMON

These new modules will be required by xt_LOG and nft_log. This patch
is based on original patch from Arturo Borrero Gonzalez.

Signed-off-by: Pablo Neira Ayuso <[email protected]>
diff --git a/net/netfilter/Makefile b/net/netfilter/Makefile
index bffdad7..8308624 100644
--- a/net/netfilter/Makefile
+++ b/net/netfilter/Makefile
@@ -47,6 +47,9 @@
 nf_nat-y	:= nf_nat_core.o nf_nat_proto_unknown.o nf_nat_proto_common.o \
 		   nf_nat_proto_udp.o nf_nat_proto_tcp.o nf_nat_helper.o
 
+# generic transport layer logging
+obj-$(CONFIG_NF_LOG_COMMON) += nf_log_common.o
+
 obj-$(CONFIG_NF_NAT) += nf_nat.o
 
 # NAT protocols (nf_nat)