powerpc/perf: Fix IMC initialization crash

Panic observed with latest firmware, and upstream kernel:

 NIP init_imc_pmu+0x8c/0xcf0
 LR  init_imc_pmu+0x2f8/0xcf0
 Call Trace:
   init_imc_pmu+0x2c8/0xcf0 (unreliable)
   opal_imc_counters_probe+0x300/0x400
   platform_drv_probe+0x64/0x110
   driver_probe_device+0x3d8/0x580
   __driver_attach+0x14c/0x1a0
   bus_for_each_dev+0x8c/0xf0
   driver_attach+0x34/0x50
   bus_add_driver+0x298/0x350
   driver_register+0x9c/0x180
   __platform_driver_register+0x5c/0x70
   opal_imc_driver_init+0x2c/0x40
   do_one_initcall+0x64/0x1d0
   kernel_init_freeable+0x280/0x374
   kernel_init+0x24/0x160
   ret_from_kernel_thread+0x5c/0x74

While registering nest imc at init, cpu-hotplug callback
nest_pmu_cpumask_init() makes an OPAL call to stop the engine. And if
the OPAL call fails, imc_common_cpuhp_mem_free() is invoked to cleanup
memory and cpuhotplug setup.

But when cleaning up the attribute group, we are dereferencing the
attribute element array without checking whether the...