powerpc/perf: Fix kfree memory allocated for nest pmus

imc_common_cpuhp_mem_free() is the common function for all
IMC (In-memory Collection counters) domains to unregister cpuhotplug
callback and free memory. Since kfree of memory allocated for
nest-imc (per_nest_pmu_arr) is in the common code, all
domains (core/nest/thread) can do the kfree in the failure case.

This could potentially create a call trace as shown below, where
core(/thread/nest) imc pmu initialization fails and in the failure
path imc_common_cpuhp_mem_free() free the memory(per_nest_pmu_arr),
which is allocated by successfully registered nest units.

The call trace is generated in a scenario where core-imc
initialization is made to fail and a cpuhotplug is performed in a p9
system. During cpuhotplug ppc_nest_imc_cpu_offline() tries to access
per_nest_pmu_arr, which is already freed by core-imc.

  NIP [c000000000cb6a94] mutex_lock+0x34/0x90
  LR [c000000000cb6a88] mutex_lock+0x28/0x90
  Call Trace:
    mutex_lock+0x28/0x90 (unreliable)
    perf_pmu_migrate_context+0x90/0x3a0
    ppc_nest_imc_cpu_offline+0x190/0x1f0
    cpuhp_invoke_callback+0x160/0x820
    cpuhp_thread_fun+0x1bc/0x270
    smpboot_thread_fn+0x250/0x290
    kthread+0x1a8/0x1b0
    ret_from_kernel_thread+0x5c/0x74

To address this scenario do the kfree(per_nest_pmu_arr) only in case
of nest-imc initialization failure, and when there is no other nest
units registered.

Fixes: 73ce9aec

 ("powerpc/perf: Fix IMC_MAX_PMU macro")
Signed-off-by: Anju T Sudhakar <anju@linux.vnet.ibm.com>
Reviewed-by: Madhavan Srinivasan <maddy@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>