Tighten valgrind integration.

Tighten valgrind integration such that immediately after memory is
validated or zeroed, valgrind is told to forget the memory's 'defined'
state.  The only place newly allocated memory should be left marked as
'defined' is in the public functions (e.g. calloc() and realloc()).

src/chunk_dss.c

@@ -127,6 +127,7 @@ chunk_alloc_dss(size_t size, size_t alignment, bool *zero)
 				if (*zero) {
 					VALGRIND_MAKE_MEM_UNDEFINED(ret, size);
 					memset(ret, 0, size);
+					VALGRIND_MAKE_MEM_UNDEFINED(ret, size);
 				}
 				return (ret);
 			}