bcache: Improve bucket_prio() calculation

When deciding what order to reuse buckets we take into account both the bucket's
priority (which indicates lru order) and also the amount of live data in that
bucket. The way they were scaled together wasn't as correct as it could be...
this patch improves and documents it.

Signed-off-by: Kent Overstreet <kmo@daterainc.com>
diff --git a/drivers/md/bcache/bcache.h b/drivers/md/bcache/bcache.h
index 93b8484..0c707e4 100644
--- a/drivers/md/bcache/bcache.h
+++ b/drivers/md/bcache/bcache.h
@@ -704,7 +704,7 @@
 };
 
 #define BTREE_PRIO		USHRT_MAX
-#define INITIAL_PRIO		32768
+#define INITIAL_PRIO		32768U
 
 #define btree_bytes(c)		((c)->btree_pages * PAGE_SIZE)
 #define btree_blocks(b)							\