On a system with only one CPU online, when another one CPU panics while
starting-up, smp_send_stop() will fail to send any STOP message to the
other already online core, resulting in a system still responsive and
alive at the end of the panic procedure.

[  186.700083] CPU3: shutdown
[  187.075462] CPU2: shutdown
[  187.162869] CPU1: shutdown
[  188.689998] ------------[ cut here ]------------
[  188.691645] kernel BUG at arch/arm64/kernel/cpufeature.c:886!
[  188.692079] Internal error: Oops - BUG: 0 [#1] PREEMPT SMP
[  188.692444] Modules linked in:
[  188.693031] CPU: 3 PID: 0 Comm: swapper/3 Not tainted 5.6.0-rc4-00001-g338d25c35a98 #104
[  188.693175] Hardware name: Foundation-v8A (DT)
[  188.693492] pstate: 200001c5 (nzCv dAIF -PAN -UAO)
[  188.694183] pc : has_cpuid_feature+0xf0/0x348
[  188.694311] lr : verify_local_elf_hwcaps+0x84/0xe8
[  188.694410] sp : ffff800011b1bf60
[  188.694536] x29: ffff800011b1bf60 x28: 0000000000000000
[  188.694707] x27: 0000000000000000 x26: 0000000000000000
[  188.694801] x25: 0000000000000000 x24: ffff80001189a25c
[  188.694905] x23: 0000000000000000 x22: 0000000000000000
[  188.694996] x21: ffff8000114aa018 x20: ffff800011156a38
[  188.695089] x19: ffff800010c944a0 x18: 0000000000000004
[  188.695187] x17: 0000000000000000 x16: 0000000000000000
[  188.695280] x15: 0000249dbde5431e x14: 0262cbe497efa1fa
[  188.695371] x13: 0000000000000002 x12: 0000000000002592
[  188.695472] x11: 0000000000000080 x10: 00400032b5503510
[  188.695572] x9 : 0000000000000000 x8 : ffff800010c80204
[  188.695659] x7 : 00000000410fd0f0 x6 : 0000000000000001
[  188.695750] x5 : 00000000410fd0f0 x4 : 0000000000000000
[  188.695836] x3 : 0000000000000000 x2 : ffff8000100939d8
[  188.695919] x1 : 0000000000180420 x0 : 0000000000180480
[  188.696253] Call trace:
[  188.696410]  has_cpuid_feature+0xf0/0x348
[  188.696504]  verify_local_elf_hwcaps+0x84/0xe8
[  188.696591]  check_local_cpu_capabilities+0x44/0x128
[  188.696666]  secondary_start_kernel+0xf4/0x188
[  188.697150] Code: 52805001 72a00301 6b01001f 54000ec0 (d4210000)
[  188.698639] ---[ end trace 3f12ca47652f7b72 ]---
[  188.699160] Kernel panic - not syncing: Attempted to kill the idle task!
[  188.699546] Kernel Offset: disabled
[  188.699828] CPU features: 0x00004,20c02008
[  188.700012] Memory Limit: none
[  188.700538] ---[ end Kernel panic - not syncing: Attempted to kill the idle task! ]---

[root@arch ~]# echo Helo
Helo
[root@arch ~]# cat /proc/cpuinfo | grep proce
processor	: 0

Make smp_send_stop() account also for the online status of the calling CPU
while evaluating how many CPUs are effectively online: this way, the right
number of STOPs is sent, so enforcing a proper freeze of the system at the
end of panic even under the above conditions.

Fixes: 08e875c1 ("arm64: SMP support")
Reported-by: Dave Martin <Dave.Martin@arm.com>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>