staging: lustre: remove back_to_sleep()

When 'back_to_sleep()' is passed as the 'timeout' function,
the effect is to wait indefinitely for the event, polling
once after the timeout.
If LWI_ON_SIGNAL_NOOP is given, then after the timeout
we allow fatal signals to interrupt the wait.

Make this more obvious in both places "back_to_sleep()" is
used but using two explicit sleeps.

The code in ptlrpcd_add_req() looks odd - why not just have one
wait_event_idle()?  However I believe this is a faithful
transformation of the existing code.

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>