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