Support -D_FORTIFY_SOURCE=3 by using __builtin_dynamic_object_size.

As explained in the issue, -D_FORTIFY_SOURCE=3 requires usage
of __builtin_dynamic_object_size in MALLOC_SIZEOF_SAFE macro.

Fixes: systemd#22801

src/basic/alloc-util.h

@@ -174,13 +174,19 @@ void* greedy_realloc0(void **p, size_t need, size_t size);
  * is compatible with _FORTIFY_SOURCES. If _FORTIFY_SOURCES is used many memory operations will take the
  * object size as returned by __builtin_object_size() into account. Hence, let's return the smaller size of
  * malloc_usable_size() and __builtin_object_size() here, so that we definitely operate in safe territory by
- * both the compiler's and libc's standards. Note that __builtin_object_size() evaluates to SIZE_MAX if the
- * size cannot be determined, hence the MIN() expression should be safe with dynamically sized memory,
- * too. Moreover, when NULL is passed malloc_usable_size() is documented to return zero, and
+ * both the compiler's and libc's standards. Note that _FORTIFY_SOURCES=3 handles also dynamically allocated
+ * objects and thus it's safer using __builtin_dynamic_object_size if _FORTIFY_SOURCES=3 is used (#22801).
+ * Moreover, when NULL is passed malloc_usable_size() is documented to return zero, and
  * __builtin_object_size() returns SIZE_MAX too, hence we also return a sensible value of 0 in this corner
  * case. */
+
+#if defined __has_builtin && __has_builtin(__builtin_dynamic_object_size)
+#define MALLOC_SIZEOF_SAFE(x) \
+        MIN(malloc_usable_size(x), __builtin_dynamic_object_size(x, 0))
+#else
 #define MALLOC_SIZEOF_SAFE(x) \
         MIN(malloc_usable_size(x), __builtin_object_size(x, 0))
+#endif
 
 /* Inspired by ELEMENTSOF() but operates on malloc()'ed memory areas: typesafely returns the number of items
  * that fit into the specified memory block */