selftests/net: fix a char signedness issue

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 <willemb@google.com>
Link: https://lore.kernel.org/r/20231124171645.1011043-3-willemdebruijn.kernel@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>