libbpf: Switch to void * casting in netlink helpers

Netlink helpers I added in 8bbb77b7c7a2 ("libbpf: Add various netlink
helpers") used char * casts everywhere, and there were a few more that
existed from before.

Convert all of them to void * cast, as it is treated equivalently by
clang/gcc for the purposes of pointer arithmetic and to follow the
convention elsewhere in the kernel/libbpf.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20210619041454.417577-2-memxor@gmail.com

src/nlattr.c

@@ -27,7 +27,7 @@ static struct nlattr *nla_next(const struct nlattr *nla, int *remaining)
 	int totlen = NLA_ALIGN(nla->nla_len);
 
 	*remaining -= totlen;
-	return (struct nlattr *) ((char *) nla + totlen);
+	return (struct nlattr *)((void *)nla + totlen);
 }
 
 static int nla_ok(const struct nlattr *nla, int remaining)