KVM: x86: Optimize kvm->lock and SRCU interaction (KVM_SET_PMU_EVENT_FILTER)

Reduce time spent holding kvm->lock: unlock mutex before calling
synchronize_srcu_expedited().  There is no need to hold kvm->lock until
all vCPUs have been kicked, KVM only needs to guarantee that all vCPUs
will switch to the new filter before exiting to userspace.  Protecting
the write to __reprogram_pmi is also unnecessary as a vCPU may process
a set bit before receiving the final KVM_REQ_PMU, but the per-vCPU writes
are guaranteed to occur after all vCPUs have switched to the new filter.

Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Michal Luczaj <mhal@rbox.co>
Link: https://lore.kernel.org/r/20230107001256.2365304-2-mhal@rbox.co


[sean: expand changelog]
Signed-off-by: Sean Christopherson <seanjc@google.com>