perf: Clean up mediated vPMU accounting

The mediated_pmu_account_event() and perf_create_mediated_pmu()
functions implement the exclusion between '!exclude_guest' counters
and mediated vPMUs. Their implementation is basically identical,
except mirrored in what they count/check.

Make sure the actual implementations reflect this similarity.

Notably:
 - while perf_release_mediated_pmu() has an underflow check;
   mediated_pmu_unaccount_event() did not.
 - while perf_create_mediated_pmu() has an inc_not_zero() path;
   mediated_pmu_account_event() did not.

Also, the inc_not_zero() path can be outsite of
perf_mediated_pmu_mutex. The mutex must guard the 0->1 (of either
nr_include_guest_events or nr_mediated_pmu_vms) transition, but once a
counter is already non-zero, it can safely be incremented further.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/20251208115156.GE3707891@noisy.programming.kicks-ass.net

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