locking/rtmutex: Consolidate rt_mutex_init()

rt_mutex_init() only initializes lockdep if CONFIG_DEBUG_RT_MUTEXES is
enabled, which is fine because all lockdep variants select it, but there is
no reason to do so.

Move the function outside of the CONFIG_DEBUG_RT_MUTEXES block which
removes #ifdeffery.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20210326153943.437405350@linutronix.de