selftests/net: fix a char signedness issue

stable inclusion
from stable-v6.6.5
commit aea066ac9e9a4cf246cf8d63a160fc637ab38244
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I8N21P

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=aea066ac9e9a4cf246cf8d63a160fc637ab38244

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[ Upstream commit 7b29828c5af6841bdeb9fafa32fdfeff7ab9c407 ]

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: de17e305 ("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>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>