staging: lustre: replace simple cases of l_wait_event() with wait_event().

When the lwi arg is full of zeros, l_wait_event() behaves almost
identically to the standard wait_event_idle() interface, so use that
instead.

l_wait_event() uses TASK_INTERRUPTIBLE, but blocks all signals.
wait_event_idle() uses the new TASK_IDLE and so avoids adding
to the load average without needing to block signals.

In one case, wait_event_idle_exclusive() is needed.

Also remove all l_wait_condition*() macros which were short-cuts
for setting lwi to {0}.

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>