bcache: use kmemdup_nul for CACHED_LABEL buffer

This patch uses kmemdup_nul to create a NUL-terminated string from
dc->sb.label. This is better than open coding it.

With this, we can move env[2] initialization into env[] array to make
code more elegant.

Signed-off-by: Geliang Tang <geliangtang@gmail.com>
Signed-off-by: Coly Li <colyli@suse.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>