Fix incompatible pointer to integer conversion initializing

[listout]

Makes struct members explicit.

First found on Gentoo linux, with MUSL LLVM profile. Most probably due to newer compilers (like Clang 16 and GCC 14) have turned various errors on by default and we get build errors such as:

net.c:115:79: error: incompatible pointer to integer conversion initializing 'int' with an expression of type 'void *' [-Wint-conversion]
        struct msghdr           s_msg = { (void *)&s_sa, sizeof(struct sockaddr_nl), s_io, 1, NULL, 0, 0};
                                                                                              ^~~~
/usr/include/unistd.h:25:14: note: expanded from macro 'NULL'
             ^~~~~~~~~~
net.c:121:79: error: incompatible pointer to integer conversion initializing 'int' with an expression of type 'void *' [-Wint-conversion]
        struct msghdr           r_msg = { (void *)&r_sa, sizeof(struct sockaddr_nl), r_io, 1, NULL, 0, 0};
                                                                                              ^~~~
/usr/include/unistd.h:25:14: note: expanded from macro 'NULL'

Bug: https://bugs.gentoo.org/897840