KVM: x86: Push acquisition of SRCU in fastpath into kvm_pmu_trigger_event()

Acquire SRCU in the VM-Exit fastpath if and only if KVM needs to check the
PMU event filter, to further trim the amount of code that is executed with
SRCU protection in the fastpath.  Counter-intuitively, holding SRCU can do
more harm than good due to masking potential bugs, and introducing a new
SRCU-protected asset to code reachable via kvm_skip_emulated_instruction()
would be quite notable, i.e. definitely worth auditing.

E.g. the primary user of kvm->srcu is KVM's memslots, accessing memslots
all but guarantees guest memory may be accessed, accessing guest memory
can fault, and page faults might sleep, which isn't allowed while IRQs are
disabled.  Not acquiring SRCU means the (hypothetical) illegal sleep would
be flagged when running with PROVE_RCU=y, even if DEBUG_ATOMIC_SLEEP=n.

Note, performance is NOT a motivating factor, as SRCU lock/unlock only
adds ~15 cycles of latency to fastpath VM-Exits.  I.e. overhead isn't a
concern _if_ SRCU protection needs to be extended beyond PMU events, e.g.
to honor userspace MSR filters.

Reviewed-by: Dapeng Mi <dapeng1.mi@linux.intel.com>
Link: https://lore.kernel.org/r/20250805190526.1453366-18-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

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