net: Fix minor code bug in timestamping.txt

Passing (void*)val instead of &val would make a pointer out of an integer
and cause sock_setsockopt to -EFAULT.

See tools/testing/selftests/networking/timestamping/timestamping.c
for a working example.

Cc: David S. Miller <davem@davemloft.net>
Cc: netdev@vger.kernel.org
Signed-off-by: Ahmad Fatoum <ahmad@a3f.at>
Signed-off-by: David S. Miller <davem@davemloft.net>