netfilter: Avoid assigning 'const' pointer to non-const pointer

nf_remove_net_hook() uses WRITE_ONCE() to assign a 'const' pointer to a
'non-const' pointer. Cleanups to the implementation of WRITE_ONCE() mean
that this will give rise to a compiler warning, just like a plain old
assignment would do:

  | In file included from ./include/linux/export.h:43,
  |                  from ./include/linux/linkage.h:7,
  |                  from ./include/linux/kernel.h:8,
  |                  from net/netfilter/core.c:9:
  | net/netfilter/core.c: In function ‘nf_remove_net_hook’:
  | ./include/linux/compiler.h:216:30: warning: assignment discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
  |   *(volatile typeof(x) *)&(x) = (val);  \
  |                               ^
  | net/netfilter/core.c:379:3: note: in expansion of macro ‘WRITE_ONCE’
  |    WRITE_ONCE(orig_ops[i], &dummy_ops);
  |    ^~~~~~~~~~

Follow the pattern used elsewhere in this file and add a cast to 'void *'
to squash the warning.

Cc: Pablo Neira Ayuso <pablo@netfilter.org>
Cc: Jozsef Kadlecsik <kadlec@netfilter.org>
Cc: Florian Westphal <fw@strlen.de>
Cc: "David S. Miller" <davem@davemloft.net>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Will Deacon <will@kernel.org>