Fix another stray "msort()/msortv()" issue

Summary: Ref T13303. See B22967. This should be "msortv()" but didn't get updated properly.

Test Plan: The system works!

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13303

Differential Revision: https://secure.phabricator.com/D20585