net: Add comment that early_demux can change via sysctl

Twice patches trying to constify inet{6}_protocol have been reverted:
39294c3d ("Revert "ipv6: constify inet6_protocol structures"") to
revert 3a3a4e30 and then 03157937 ("Revert "ipv4: make
net_protocol const"") to revert aa8db499.

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>