powerpc: Fix stack protector crashes on CPU hotplug

Recently in commit 7241d26e ("powerpc/64: properly initialise
the stackprotector canary on SMP.") we fixed a crash with stack
protector on SMP by initialising the stack canary in
cpu_idle_thread_init().

But this can also causes crashes, when a CPU comes back online after
being offline:

  Kernel panic - not syncing: stack-protector: Kernel stack is corrupted in: pnv_smp_cpu_kill_self+0x2a0/0x2b0
  CPU: 1 PID: 0 Comm: swapper/1 Not tainted 4.19.0-rc3-gcc-7.3.1-00168-g4ffe713b7587 #94
  Call Trace:
    dump_stack+0xb0/0xf4 (unreliable)
    panic+0x144/0x328
    __stack_chk_fail+0x2c/0x30
    pnv_smp_cpu_kill_self+0x2a0/0x2b0
    cpu_die+0x48/0x70
    arch_cpu_idle_dead+0x20/0x40
    do_idle+0x274/0x390
    cpu_startup_entry+0x38/0x50
    start_secondary+0x5e4/0x600
    start_secondary_prolog+0x10/0x14

Looking at the stack we see that the canary value in the stack frame
doesn't match the canary in the task/paca. That is because we have
reinitialised the task/paca value, but then the CPU coming online has
returned into a function using the old canary value. That causes the
comparison to fail.

Instead we can call boot_init_stack_canary() from start_secondary()
which never returns. This is essentially what the generic code does in
cpu_startup_entry() under #ifdef X86, we should make that non-x86
specific in a future patch.

Fixes: 7241d26e

 ("powerpc/64: properly initialise the stackprotector canary on SMP.")
Reported-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Reviewed-by: Christophe Leroy <christophe.leroy@c-s.fr>