Revert "locking/rtmutex: Fix task->pi_waiters integrity"

This reverts commit 4ed15491 which is
commit f7853c34 upstream.

It breaks the Android api due to a structure, struct rt_mutex_waiter,
escaping out of the core kernel code, into an Android hook, making it
something that can not be modified without potentially major problems.

If this needs to come back, it must do so in an ABI-safe way in the
future, for now, it can just be reverted.

Bug: 161946584
Change-Id: Idd251aa4e905cc284523b0b81b3e8993c6a58866
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>