[space-map] Add new virtual methods to inc or dec by a specific amount.

Provide default implementation that just loops

persistent-data/space_map.h

@@ -48,6 +48,17 @@ namespace persistent_data {
 		virtual void inc(block_address b) = 0;
 		virtual void dec(block_address b) = 0;
 
+		// slow default implementation
+		virtual void inc(block_address b, uint32_t count) {
+			for (uint32_t i = 0; i < count; i++)
+				inc(b);
+		}
+
+		virtual void dec(block_address b, uint32_t count) {
+			for (uint32_t i = 0; i < count; i++)
+				dec(b);
+		}
+
 		// FIXME: change these to return an optional, failure is
 		// not that rare if we're restricting the area that's
 		// searched.