Fix a typo introduced in r344133

The line was misedited to change tt to st instead of
changing ut to st.

The use of st as the denominator in mul64_by_fraction() will lead
to an integer divide fault in the intr proc (the process holding
ithreads) where st will be 0. This divide by 0 happens after
the total runtime for all ithreads exceeds 76 hours.

Submitted by: bde