locking/mutex: Move the 'struct mutex_waiter' definition from <linux/mutex.h>...

locking/mutex: Move the 'struct mutex_waiter' definition from <linux/mutex.h> to the internal header

Move the mutex waiter declaration from the public <linux/mutex.h> header
to the internal kernel/locking/mutex.h header.

There is no reason to expose it outside of the core code.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20210815211304.054325923@linutronix.de