turnstile_lock() was added specifically for use in epoch. While waiting
for readers to drain, if the blocking thread discovers a reader blocked
on a turnstile, it will also block on the turnstile. It previously
returned the value of the ts_owner field to indicate that the turnstile
was successfully locked, and NULL to indicate that the race was lost.
But, ts_owner may be NULL if the turnstile corresponds to a rw lock
owned by one or more readers, in which case the epoch code leaked the
turnstile lock and chain lock.
Apply a minimal fix: simply return the owner as a separate value.