fellow_cache: Fix bad cast in recent io failure reporting code

Found by flexelint

Ref a9700c63
Ref #65

src/fellow_cache.c

@@ -2219,7 +2219,7 @@ fellow_cache_seglists_load(struct worker *wrk, void *priv)
 		ssz = fellow_io_pread_sync(fc->ffd, fdsl, next.size, next.off);
 		if (FC_INJ || ssz < 0 || (size_t)ssz != next.size) {
 			fcr = FCR_IOFAILHOW("disk seglist read",
-				(size_t)ssz, next.size);
+				ssz, next.size);
 			goto err;
 		}
 		err = fellow_disk_seglist_check(fdsl);