bpftool: Fix a clang compilation warning

With clang compiler:
  make -j60 LLVM=1 LLVM_IAS=1  <=== compile kernel
  # build selftests/bpf or bpftool
  make -j60 -C tools/testing/selftests/bpf LLVM=1 LLVM_IAS=1
  make -j60 -C tools/bpf/bpftool LLVM=1 LLVM_IAS=1
the following compilation warning showed up,
  net.c:160:37: warning: comparison of integers of different signs: '__u32' (aka 'unsigned int') and 'int' [-Wsign-compare]
                for (nh = (struct nlmsghdr *)buf; NLMSG_OK(nh, len);
                                                  ^~~~~~~~~~~~~~~~~
  .../tools/include/uapi/linux/netlink.h:99:24: note: expanded from macro 'NLMSG_OK'
                           (nlh)->nlmsg_len <= (len))
                           ~~~~~~~~~~~~~~~~ ^   ~~~

In this particular case, "len" is defined as "int" and (nlh)->nlmsg_len is "unsigned int".
The macro NLMSG_OK is defined as below in uapi/linux/netlink.h.
  #define NLMSG_OK(nlh,len) ((len) >= (int)sizeof(struct nlmsghdr) && \
                             (nlh)->nlmsg_len >= sizeof(struct nlmsghdr) && \
                             (nlh)->nlmsg_len <= (len))

The clang compiler complains the comparision "(nlh)->nlmsg_len <= (len))",
but in bpftool/net.c, it is already ensured that "len > 0" must be true.
So theoretically the compiler could deduce that comparison of
"(nlh)->nlmsg_len" and "len" is okay, but this really depends on compiler
internals. Let us add an explicit type conversion (from "int" to "unsigned int")
for "len" in NLMSG_OK to silence this warning right now.
Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210413153435.3029635-1-yhs@fb.com

tools/bpf/bpftool/net.c

@@ -157,7 +157,7 @@ static int netlink_recv(int sock, __u32 nl_pid, __u32 seq,
 		if (len == 0)
 			break;
 
-		for (nh = (struct nlmsghdr *)buf; NLMSG_OK(nh, len);
+		for (nh = (struct nlmsghdr *)buf; NLMSG_OK(nh, (unsigned int)len);
 		     nh = NLMSG_NEXT(nh, len)) {
 			if (nh->nlmsg_pid != nl_pid) {
 				ret = -LIBBPF_ERRNO__WRNGPID;