kernel/futex: Explicitly document pi_lock for pi_state owner fixup

This seems to belong in the serialization and lifetime rules section.
pi_state_update_owner() will take the pi_mutex's owner's pi_lock to
do whatever fixup, successful or not.

Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20210226175029.50335-4-dave@stgolabs.net