sched/wait: Standardize internal naming of wait-queue entries

sched/wait: Standardize internal naming of wait-queue entries

So the various wait-queue entry variables in include/linux/wait.h
and kernel/sched/wait.c are named in a colorfully inconsistent
way:

	wait_queue_entry_t *wait
	wait_queue_entry_t *__wait	(even in plain C code!)
	wait_queue_entry_t *q		(!)
	wait_queue_entry_t *new		(making anyone who knows C++ cringe)
	wait_queue_entry_t *old

I think part of the reason for the inconsistency is the constant
apparent confusion about what a wait queue 'head' versus 'entry' is.

( Some of the documentation talks about a 'wait descriptor', which is
  the wait-queue entry itself - further adding to the confusion. )

The most common name is 'wait', but that in itself is somewhat
ambiguous as well, as it does not really make it clear whether
it's a wait-queue entry or head.

To improve all this name the wait-queue entry structure parameters
and variables consistently and push through this naming into all
the wait.h and wait.c code:

	struct wait_queue_entry *wq_entry

The 'wq_' prefix makes it easy to grep for, and we also use the
opportunity to move away from the typedef to a plain 'struct' naming:
in the kernel we typically reserve typedefs for cases where a
C structure is really small and somewhat opaque - such as pte_t.

wait-queue entries are neither small nor opaque, so use the more
standard 'struct xxx_entry' list management code nomenclature instead.

( We don't touch external users, and we preserve the typedef as well
  for actual wait-queue users, to reduce unnecessary churn. )

Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>