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 <[e-mail]>
Signed-off-by: Thomas Gleixner <[e-mail]>
Acked-by: Peter Zijlstra (Intel) <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

kernel/futex.c

···
  * p->pi_lock:
  *
  *	p->pi_state_list -> pi_state->list, relation
+ *	pi_mutex->owner -> pi_state->owner, relation
  *
  * pi_state->refcount:
  *