rtmutex: Add rt_mutex_lock_nest_lock() and rt_mutex_lock_killable().

The locking selftest for ww-mutex expects to operate directly on the
base-mutex which becomes a rtmutex on PREEMPT_RT.

Add rt_mutex_lock_nest_lock(), follows mutex_lock_nest_lock() for
rtmutex.
Add rt_mutex_lock_killable(), follows mutex_lock_killable() for rtmutex.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>