bcache-tools are GPL
fix make-bcache to use logical_block_size, not stat.st_blocksize

As discussed on irc yesterday.

Cheers,
	Lars

From 791a4b7b2c25e21ffeb4184da5e61f18cde86246 Mon Sep 17 00:00:00 2001
From: Lars Ellenberg <lars@linbit.com>
Date: Wed, 17 Jul 2013 11:49:12 +0200
Subject: [PATCH] make-bcache: fix guessing of "block_size" to use BLKSSZGET

Using stat.st_blocksize is wrong:
that is the linux buffer cache granularity,
it is tunable, and defaults to 4k.

This caused bcache to default to create devices with
logical_block_size = 4k, potentially breaking otherwise healthy disk
or file system images unnecessarily.

e.g. some xfs image previously created with 512 byte "sector size"
would, once bcache'd, refuse to mount with
	mount: function not implemented
	device supports 4096 byte sectors (not 512)

We want the logical_block_size here.
Don't register if a non-bcache superblock is found as well