net: Add comment that early_demux can change via sysctl

net: Add comment that early_demux can change via sysctl

Twice patches trying to constify inet{6}_protocol have been reverted:
99e2f995f65c ("Revert "ipv6: constify inet6_protocol structures"") to
revert 5ba9116428d6f and then 58622aeced2a0 ("Revert "ipv4: make
net_protocol const"") to revert fbdfce42743e.

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>