locking/rwsem: Add missing __init_rwsem() for PREEMPT_RT

730633f0 became the first direct caller of __init_rwsem() vs the
usual init_rwsem(), exposing PREEMPT_RT's lack thereof.  Add it.

[ tglx: Move it out of line ]

Signed-off-by: Mike Galbraith <efault@gmx.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/50a936b7d8f12277d6ec7ed2ef0421a381056909.camel@gmx.de