bcachefs: Make open bucket reserves more conservative

Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/alloc_foreground.c

@@ -212,9 +212,9 @@ static inline unsigned open_buckets_reserved(enum alloc_reserve reserve)
 	case RESERVE_ALLOC:
 		return 0;
 	case RESERVE_BTREE:
-		return BTREE_NODE_OPEN_BUCKET_RESERVE;
+		return OPEN_BUCKETS_COUNT / 4;
 	default:
-		return BTREE_NODE_OPEN_BUCKET_RESERVE * 2;
+		return OPEN_BUCKETS_COUNT / 2;
 	}
 }