kexec: use atomic_try_cmpxchg in crash_kexec

Use atomic_try_cmpxchg instead of cmpxchg (*ptr, old, new) == old in
crash_kexec().  x86 CMPXCHG instruction returns success in ZF flag,
so this change saves a compare after cmpxchg.

No functional change intended.

Link: https://lkml.kernel.org/r/20231114161228.108516-1-ubizjak@gmail.comSigned-off-by: Uros Bizjak <ubizjak@gmail.com>
Acked-by: Baoquan He <bhe@redhat.com>
Cc: Eric Biederman <ebiederm@xmission.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

kernel/kexec_core.c

@@ -1063,9 +1063,10 @@ __bpf_kfunc void crash_kexec(struct pt_regs *regs)
 	 * panic().  Otherwise parallel calls of panic() and crash_kexec()
 	 * may stop each other.  To exclude them, we use panic_cpu here too.
 	 */
+	old_cpu = PANIC_CPU_INVALID;
 	this_cpu = raw_smp_processor_id();
-	old_cpu = atomic_cmpxchg(&panic_cpu, PANIC_CPU_INVALID, this_cpu);
-	if (old_cpu == PANIC_CPU_INVALID) {
+
+	if (atomic_try_cmpxchg(&panic_cpu, &old_cpu, this_cpu)) {
 		/* This is the 1st CPU which comes here, so go ahead. */
 		__crash_kexec(regs);