KVM: arm64: Use raw_smp_processor_id() in kvm_pmu_probe_armpmu()

KVM: arm64: Use raw_smp_processor_id() in kvm_pmu_probe_armpmu()

Sebastian reports that commit 1c913a1c35aa ("KVM: arm64: Iterate
arm_pmus list to probe for default PMU") introduced the following splat
with CONFIG_DEBUG_PREEMPT enabled:

[70506.110187] BUG: using smp_processor_id() in preemptible [00000000] code: qemu-system-aar/3078242
[70506.119077] caller is debug_smp_processor_id+0x20/0x30
[70506.124229] CPU: 129 PID: 3078242 Comm: qemu-system-aar Tainted: G        W          6.4.0-rc5 #25
[70506.133176] Hardware name: GIGABYTE R181-T92-00/MT91-FS4-00, BIOS F34 08/13/2020
[70506.140559] Call trace:
[70506.142993]  dump_backtrace+0xa4/0x130
[70506.146737]  show_stack+0x20/0x38
[70506.150040]  dump_stack_lvl+0x48/0x60
[70506.153704]  dump_stack+0x18/0x28
[70506.157007]  check_preemption_disabled+0xe4/0x108
[70506.161701]  debug_smp_processor_id+0x20/0x30
[70506.166046]  kvm_arm_pmu_v3_set_attr+0x460/0x628
[70506.170662]  kvm_arm_vcpu_arch_set_attr+0x88/0xd8
[70506.175363]  kvm_arch_vcpu_ioctl+0x258/0x4a8
[70506.179632]  kvm_vcpu_ioctl+0x32c/0x6b8
[70506.183465]  __arm64_sys_ioctl+0xb4/0x100
[70506.187467]  invoke_syscall+0x78/0x108
[70506.191205]  el0_svc_common.constprop.0+0x4c/0x100
[70506.195984]  do_el0_svc+0x34/0x50
[70506.199287]  el0_svc+0x34/0x108
[70506.202416]  el0t_64_sync_handler+0xf4/0x120
[70506.206674]  el0t_64_sync+0x194/0x198

Fix the issue by using the raw variant that bypasses the debug
assertion. While at it, stick all of the nuance and UAPI baggage into a
comment for posterity.

Fixes: 1c913a1c35aa ("KVM: arm64: Iterate arm_pmus list to probe for default PMU")
Reported-by: Sebastian Ott <sebott@redhat.com>
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20230606184814.456743-1-oliver.upton@linux.dev