smp: kernel/panic.c - silence warnings

We found these warnings in kernel/panic.c by using sparse tool:
warning: symbol 'panic_smp_self_stop' was not declared.
warning: symbol 'nmi_panic_self_stop' was not declared.
warning: symbol 'crash_smp_send_stop' was not declared.

To avoid them, add declarations for these three functions in
include/linux/smp.h.

Link: https://lkml.kernel.org/r/20210316084150.75201-1-heying24@huawei.comSigned-off-by: He Ying <heying24@huawei.com>
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/smp.h

@@ -55,6 +55,14 @@ void on_each_cpu_cond_mask(smp_cond_func_t cond_func, smp_call_func_t func,
 
 int smp_call_function_single_async(int cpu, call_single_data_t *csd);
 
+/*
+ * Cpus stopping functions in panic. All have default weak definitions.
+ * Architecture-dependent code may override them.
+ */
+void panic_smp_self_stop(void);
+void nmi_panic_self_stop(struct pt_regs *regs);
+void crash_smp_send_stop(void);
+
 /*
  * Call a function on all processors
  */