KVM: x86: Optimize kvm->lock and SRCU interaction (KVM_X86_SET_MSR_FILTER)

Reduce time spent holding kvm->lock: unlock mutex before calling
synchronize_srcu().  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.

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/20230107001256.2365304-3-mhal@rbox.co


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