staging: lustre: make polling loop in ptlrpc_unregister_bulk more obvious

This use of l_wait_event() is a polling loop that re-checks
every second.  Make this more obvious with a while loop
and wait_event_idle_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>