bcache: Pull on disk data structures out into a separate header

Now, the on disk data structures are in a header that can be exported to
userspace - and having them all centralized is nice too.

Signed-off-by: Kent Overstreet <kmo@daterainc.com>
diff --git a/drivers/md/bcache/bset.c b/drivers/md/bcache/bset.c
index f7b5525..7b8713c 100644
--- a/drivers/md/bcache/bset.c
+++ b/drivers/md/bcache/bset.c
@@ -684,7 +684,7 @@
 	} else
 		get_random_bytes(&i->seq, sizeof(uint64_t));
 
-	i->magic	= bset_magic(b->c);
+	i->magic	= bset_magic(&b->c->sb);
 	i->version	= 0;
 	i->keys		= 0;
 
@@ -1034,7 +1034,7 @@
 		 * memcpy()
 		 */
 
-		out->magic	= bset_magic(b->c);
+		out->magic	= bset_magic(&b->c->sb);
 		out->seq	= b->sets[0].data->seq;
 		out->version	= b->sets[0].data->version;
 		swap(out, b->sets[0].data);