locking/rtmutex: Avoid pointless blk_flush_plug() invocations

With DEBUG_RT_MUTEXES enabled the fast-path rt_mutex_cmpxchg_acquire()
always fails and all lock operations take the slow path, which leads to the
invocation of blk_flush_plug() even if the lock is not contended which is
unnecessary and avoids batch processing of requests.

Provide a new helper inline rt_mutex_try_acquire() which maps to
rt_mutex_cmpxchg_acquire() in the non-debug case. For the debug case it
invokes rt_mutex_slowtrylock() which can acquire a non-contended rtmutex
under full debug coverage.

Replace the rt_mutex_cmpxchg_acquire() invocations in __rt_mutex_lock() and
__ww_rt_mutex_lock() with the new helper function, which avoid the
blk_flush_plug() for the non-contended case and preserves the debug
mechanism.

[ tglx: Created a new helper and massaged changelog ]

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Link: https://lore.kernel.org/r/20230427111937.2745231-4-bigeasy@linutronix.de