selftests/net: fix a char signedness issue

Signedness of char is signed on x86_64, but unsigned on arm64.

Fix the warning building cmsg_sender.c on signed platforms or
forced with -fsigned-char:

msg_sender.c:455:12:
error: implicit conversion from 'int' to 'char'
changes value from 128 to -128
[-Werror,-Wconstant-conversion]
buf[0] = ICMPV6_ECHO_REQUEST;

constant ICMPV6_ECHO_REQUEST is 128.

Link: https://lwn.net/Articles/911914
Fixes: de17e305a810 ("selftests: net: cmsg_sender: support icmp and raw sockets")
Signed-off-by: Willem de Bruijn <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

tools/testing/selftests/net/cmsg_sender.c

···
 {
 	struct addrinfo hints, *ai;
 	struct iovec iov[1];
+	unsigned char *buf;
 	struct msghdr msg;
 	char cbuf[1024];
-	char *buf;
 	int err;
 	int fd;