net: Add comment that early_demux can change via sysctl

Twice patches trying to constify inet{6}_protocol have been reverted:
39294c3df2a8 ("Revert "ipv6: constify inet6_protocol structures"") to
revert 3a3a4e3054137 and then 03157937fe0b5 ("Revert "ipv4: make
net_protocol const"") to revert aa8db499ea67.

Add a comment that the structures can not be const because the
early_demux field can change based on a sysctl.

Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/af_inet.c

@@ -1596,6 +1596,9 @@ static const struct net_protocol igmp_protocol = {
 };
 #endif
 
+/* thinking of making this const? Don't.
+ * early_demux can change based on sysctl.
+ */
 static struct net_protocol tcp_protocol = {
 	.early_demux	=	tcp_v4_early_demux,
 	.early_demux_handler =  tcp_v4_early_demux,
@@ -1606,6 +1609,9 @@ static struct net_protocol tcp_protocol = {
 	.icmp_strict_tag_validation = 1,
 };
 
+/* thinking of making this const? Don't.
+ * early_demux can change based on sysctl.
+ */
 static struct net_protocol udp_protocol = {
 	.early_demux =	udp_v4_early_demux,
 	.early_demux_handler =	udp_v4_early_demux,

net/ipv6/tcp_ipv6.c

@@ -1949,6 +1949,9 @@ struct proto tcpv6_prot = {
 	.diag_destroy		= tcp_abort,
 };
 
+/* thinking of making this const? Don't.
+ * early_demux can change based on sysctl.
+ */
 static struct inet6_protocol tcpv6_protocol = {
 	.early_demux	=	tcp_v6_early_demux,
 	.early_demux_handler =  tcp_v6_early_demux,

net/ipv6/udp.c

@@ -1472,6 +1472,9 @@ int compat_udpv6_getsockopt(struct sock *sk, int level, int optname,
 }
 #endif
 
+/* thinking of making this const? Don't.
+ * early_demux can change based on sysctl.
+ */
 static struct inet6_protocol udpv6_protocol = {
 	.early_demux	=	udp_v6_early_demux,
 	.early_demux_handler =  udp_v6_early_demux,