mount_fuzz: reject giant files early

It should help to address https://github.com/google/oss-fuzz/issues/6703

Signed-off-by: Evgeny Vereshchagin <evvers@ya.ru>

libmount/src/fuzz.c

@@ -13,6 +13,10 @@ int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
         if (size == 0)
                 return 0;
 
+        // 128Kb should be enough to trigger all the issues we're interested in
+        if (size > 131072)
+                return 0;
+
         tb = mnt_new_table();
         if (!tb)
 		err_oom();