powerpc/perf: Fix ref-counting on the PMU 'vpa_pmu'

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

powerpc/perf: Fix ref-counting on the PMU 'vpa_pmu'

Commit 176cda0619b6 ("powerpc/perf: Add perf interface to expose vpa
counters") introduced 'vpa_pmu' to expose Book3s-HV nested APIv2 provided
L1<->L2 context switch latency counters to L1 user-space via
perf-events. However the newly introduced PMU named 'vpa_pmu' doesn't
assign ownership of the PMU to the module 'vpa_pmu'. Consequently the
module 'vpa_pmu' can be unloaded while one of the perf-events are still
active, which can lead to kernel oops and panic of the form below on a
Pseries-LPAR:

BUG: Kernel NULL pointer dereference on read at 0x00000058
<snip>
NIP [c000000000506cb8] event_sched_out+0x40/0x258
LR [c00000000050e8a4] __perf_remove_from_context+0x7c/0x2b0
Call Trace:
[c00000025fc3fc30] [c00000025f8457a8] 0xc00000025f8457a8 (unreliable)
[c00000025fc3fc80] [fffffffffffffee0] 0xfffffffffffffee0
[c00000025fc3fcd0] [c000000000501e70] event_function+0xa8/0x120
<snip>
Kernel panic - not syncing: Aiee, killing interrupt handler!

Fix this by adding the module ownership to 'vpa_pmu' so that the module
'vpa_pmu' is ref-counted and prevented from being unloaded when perf-events
are initialized.