powerpc/kexec: Enable SMT before waking offline CPUs

In the Linux kernel, the following vulnerability has been resolved:

powerpc/kexec: Enable SMT before waking offline CPUs

If SMT is disabled or a partial SMT state is enabled, when a new kernel

image is loaded for kexec, on reboot the following warning is observed:

kexec: Waking offline cpu 228.

WARNING: CPU: 0 PID: 9062 at arch/powerpc/kexec/core_64.c:223 kexec_prepare_cpus+0x1b0/0x1bc

[snip]

NIP kexec_prepare_cpus+0x1b0/0x1bc

LR kexec_prepare_cpus+0x1a0/0x1bc

Call Trace:

kexec_prepare_cpus+0x1a0/0x1bc (unreliable)

default_machine_kexec+0x160/0x19c

machine_kexec+0x80/0x88

kernel_kexec+0xd0/0x118

__do_sys_reboot+0x210/0x2c4

system_call_exception+0x124/0x320

system_call_vectored_common+0x15c/0x2ec

This occurs as add_cpu() fails due to cpu_bootable() returning false for

CPUs that fail the cpu_smt_thread_allowed() check or non primary

threads if SMT is disabled.

Fix the issue by enabling SMT and resetting the number of SMT threads to

the number of threads per core, before attempting to wake up all present

CPUs.