bcache: Rename/shuffle various code around

More work to disentangle bset.c from the rest of the code:

Signed-off-by: Kent Overstreet <kmo@daterainc.com>
diff --git a/drivers/md/bcache/super.c b/drivers/md/bcache/super.c
index 1fc8165..12057a4 100644
--- a/drivers/md/bcache/super.c
+++ b/drivers/md/bcache/super.c
@@ -1477,7 +1477,7 @@
 	c->block_bits		= ilog2(sb->block_size);
 	c->nr_uuids		= bucket_bytes(c) / sizeof(struct uuid_entry);
 
-	c->btree_pages		= c->sb.bucket_size / PAGE_SECTORS;
+	c->btree_pages		= bucket_pages(c);
 	if (c->btree_pages > BTREE_MAX_PAGES)
 		c->btree_pages = max_t(int, c->btree_pages / 4,
 				       BTREE_MAX_PAGES);