bcache-tools: fix strncpy compiler warning in replace_line()

The strncpy() call in replace_line() was using strlen(src) as the size
parameter instead of the destination buffer size, causing a compiler
warning about potential string truncation. use snprintf() instead.

Signed-off-by: Shaoxiong Li <dahefanteng@gmail.com>
Signed-off-by: Coly Li <colyli@kernel.org>