fellow_cache: Support pointers not from fbo->region in _busy_region_free

src/fellow_cache.c

@@ -2727,10 +2727,16 @@ fellow_busy_region_free(struct fellow_busy *fbo, struct buddy_off_extent *fdr)
 	CHECK_OBJ_NOTNULL(fc, FELLOW_CACHE_MAGIC);
 
 	u = fdr - &fbo->region[0];
-	assert(u >= 0);
-	assert(u < fbo->nregion);
-
-	// obviously: assert(&fbo->region[u] == fdr);
+	if (! (u >= 0 && u < fbo->nregion)) {
+		for (u = 0; u < fbo->nregion; u++)
+			if (fbo->region[u].off == fdr->off)
+				break;
+		assert(u < fbo->nregion);
+		assert(fbo->region[u].size == fdr->size);
+		fbo->region[u] = buddy_off_extent_nil;
+	}
+	else
+		assert(&fbo->region[u] == fdr);
 
 	buddy_return1_off_extent(fellow_dskbuddy(fc->ffd), fdr);