speed up fil_validate() in debug builds

This function is very common in a debug build. I can even see it in
profiler.

This patch reduces execution time of fil_validate() from
8948ns
8367ns
8650ns
8906ns
8448ns

to
260ns
232ns
403ns
275ns
169ns
in my environment.

The trick is a faster fil_space_t iteration. Hash table
is typically initialized with a size of 50,000. And looping through
it is slow. Slower, than iterating an exact amount of fil_space_t
which is typically less than ten.

Only debug builds are affected.

storage/innobase/fil/fil0fil.cc

@@ -5314,25 +5314,16 @@ bool
 fil_validate(void)
 /*==============*/
 {
-	fil_space_t*	space;
 	fil_node_t*	fil_node;
 	ulint		n_open		= 0;
 
 	mutex_enter(&fil_system->mutex);
 
-	/* Look for spaces in the hash table */
-
-	for (ulint i = 0; i < hash_get_n_cells(fil_system->spaces); i++) {
-
-		for (space = static_cast<fil_space_t*>(
-				HASH_GET_FIRST(fil_system->spaces, i));
-		     space != 0;
-		     space = static_cast<fil_space_t*>(
-				HASH_GET_NEXT(hash, space))) {
-
+	for (fil_space_t *space = UT_LIST_GET_FIRST(fil_system->space_list);
+	     space != NULL;
+	     space = UT_LIST_GET_NEXT(space_list, space)) {
 		n_open += Check::validate(space);
 	}
-	}
 
 	ut_a(fil_system->n_open == n_open);