fork: use this_cpu_try_cmpxchg() in try_release_thread_stack_to_cache()

Use this_cpu_try_cmpxchg() instead of this_cpu_cmpxchg (*ptr, old, new) ==
old in try_release_thread_stack_to_cache.  x86 CMPXCHG instruction returns
success in ZF flag, so this change saves a compare after cmpxchg (and
related move instruction in front of cmpxchg).

No functional change intended.

[ubizjak@gmail.com: simplify the for loop a bit]
  Link: https://lkml.kernel.org/r/20240523214442.21102-1-ubizjak@gmail.com
Link: https://lkml.kernel.org/r/20240523073530.8128-1-ubizjak@gmail.comSigned-off-by: Uros Bizjak <ubizjak@gmail.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Kees Cook <keescook@chromium.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

kernel/fork.c

@@ -205,9 +205,10 @@ static bool try_release_thread_stack_to_cache(struct vm_struct *vm)
 	unsigned int i;
 
 	for (i = 0; i < NR_CACHED_STACKS; i++) {
-		if (this_cpu_cmpxchg(cached_stacks[i], NULL, vm) != NULL)
-			continue;
-		return true;
+		struct vm_struct *tmp = NULL;
+
+		if (this_cpu_try_cmpxchg(cached_stacks[i], &tmp, vm))
+			return true;
 	}
 	return false;
 }