locking/mutex: Add comment to __mutex_owner() to deter usage

locking/mutex: Add comment to __mutex_owner() to deter usage

Attempt to deter usage, this is not a public interface. It is entirely
possible to implement a conformant mutex without having this owner
field (in fact, we used to have that).

Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Paul E. McKenney <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: [email protected]
Signed-off-by: Ingo Molnar <[email protected]>