bcachefs: Heap code fix

When deleting an entry from a heap that was at entry h->used - 1, we'd
end up calling heap_sift() on an entry outside the heap - the entry we
just removed - which would end up re-adding it to the heap and deleting
something we didn't want to delete. Oops...
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>

fs/bcachefs/util.h

@@ -210,9 +210,11 @@ do {									\
 									\
 	BUG_ON(_i >= (h)->used);					\
 	(h)->used--;							\
-	heap_swap(h, _i, (h)->used, set_backpointer);			\
-	heap_sift_up(h, _i, cmp, set_backpointer);			\
-	heap_sift_down(h, _i, cmp, set_backpointer);			\
+	if ((_i) < (h)->used) {						\
+		heap_swap(h, _i, (h)->used, set_backpointer);		\
+		heap_sift_up(h, _i, cmp, set_backpointer);		\
+		heap_sift_down(h, _i, cmp, set_backpointer);		\
+	}								\
 } while (0)
 
 #define heap_pop(h, d, cmp, set_backpointer)				\