l2tp: constify inet6_protocol structures

The inet6_protocol structure is only passed as the first argument to
inet6_add_protocol or inet6_del_protocol, both of which are declared as
const.  Thus the inet6_protocol structure itself can be const.

Also drop __read_mostly on the newly const structure.

Done with the help of Coccinelle.

Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>