sched/wait: add wait_event_idle() functions.

The new TASK_IDLE state (TASK_UNINTERRUPTIBLE | __TASK_NOLOAD)
is not much used.  One way to make it easier to use is to
add wait_event*() family functions that make use of it.
This patch adds:
  wait_event_idle()
  wait_event_idle_timeout()
  wait_event_idle_exclusive()
  wait_event_idle_exclusive_timeout()

This set was chosen because lustre needs them before
it can discard its own l_wait_event() macro.

Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Signed-off-by: NeilBrown <neilb@suse.com>
Reviewed-by: Patrick Farrell <paf@cray.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>