bcache: fix indentation issue, remove tabs on a hunk of code

There is a hunk of code that is indented one level too deep, fix this
by removing the extra tabs.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Coly Li <colyli@suse.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>