fellow_cache: Fix calculation of required number of segments

it needs to be based on the space to be returned, not on the space to be still
allocated on disk.

src/fellow_cache.c

@@ -2814,7 +2814,7 @@ fellow_busy_body_seglist_alloc(struct fellow_busy *fbo,
 	 * sz is number of segments
 	 */
 	if (fbo->grown && fbo->body_seglist != fbo->fco->fcsl) {
-		sz = fbo->fc->tune->objsize_max - fbo->sz_dskalloc;
+		sz = fbo->fc->tune->objsize_max - fbo->sz_returned;
 		sz += (((size_t)1 << MIN_FELLOW_BITS) - 1);
 		sz >>= MIN_FELLOW_BITS;
 		// can not apply strategy because we could hit FCO_MAX_REGIONS