staging: lustre: replace l_wait_event_exclusive_head() with wait_event_idle_exclusive

This l_wait_event_exclusive_head() will wait indefinitely
if the timeout is zero.  If it does wait with a timeout
and times out, the timeout for next time is set to zero.

The can be mapped to a call to either
 wait_event_idle_exclusive()
or
 wait_event_idle_exclusive_timeout()
depending in the timeout setting.

The current code arranges for LIFO queuing of waiters,
but include/event.h doesn't support that yet.
Until it does, fall back on FIFO with
wait_event_idle_exclusive{,_timeout}().

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>