net/ipv6: replace deprecated strcpy with strscpy

net/ipv6: replace deprecated strcpy with strscpy

The deprecated helper strcpy() performs no bounds checking on the
destination buffer. This could result in linear overflows beyond
the end of the buffer, leading to all kinds of misbehaviors.
The safe replacement is strscpy() [1].

Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#strcpy
Signed-off-by: Hongbo Li <lihongbo22@huawei.com>
Link: https://patch.msgid.link/20240828123224.3697672-3-lihongbo22@huawei.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>