KVM: x86: Explicitly state lockdep condition of msr_filter update

Replace `1` with the actual mutex_is_locked() check.

Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Michal Luczaj <mhal@rbox.co>
Link: https://lore.kernel.org/r/20230107001256.2365304-5-mhal@rbox.co


[sean: delete the comment that explained the hardocded '1']
Signed-off-by: Sean Christopherson <seanjc@google.com>