relay: use strscpy() is more robust and safer

The implementation of strscpy() is more robust and safer.  That's now the
recommended way to copy NUL terminated strings.

Link: https://lkml.kernel.org/r/202211220853259244666@zte.com.cn


Signed-off-by: Xu Panda <xu.panda@zte.com.cn>
Signed-off-by: Yang Yang <yang.yang29@zte.com>
Cc: Colin Ian King <colin.i.king@gmail.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: wuchi <wuchi.zero@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>