Merge branch 'revert-ipv6-const'

Julia Lawall says:

====================
Revert "ipv6: constify inet6_protocol structures"

inet6_add_protocol and inet6_del_protocol include casts that remove the
effect of the const annotation on their parameter, leading to possible
runtime crashes.
====================

Signed-off-by: David S. Miller <davem@davemloft.net>