staging: lustre: simplify waiting in ptlrpc_invalidate_import()

This waiter currently wakes up every second to re-test if
imp_flight is zero.  If we ensure wakeup is called whenever
imp_flight is decremented to zero, we can just have a simple
wait_event_idle_timeout().

So add a wake_up_all to the one place it is missing, and simplify
the wait_event.

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>