staging: lustre: use explicit poll loop in ptlrpc_service_unlink_rqbd

Rather an using l_wait_event(), use wait_event_idle_timeout()
with an explicit loop so it is easier to see what is happening.

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>