LU-10467 lustre: convert users of back_to_sleep()

LU-10467 lustre: convert users of back_to_sleep()

When back_to_sleep() is passed to l_wait_event as
the on_timeout hander, the effect is to potentially wait twice.
The first wait ignores all signals and has a timeout.
If the timeout fires without the event occuring, the l_wait_event()
goes "back to sleep" indefinitely, but this time with fatal
signals unblocked.

This pattern can be made more clear with two separate wait calls:
  wait_event_idle_timeout() followed by l_wait_event_abortable().

Change-Id: I3536e33b4d982f37c960f31df1ea0d9808f9ced7
Signed-off-by: Mr NeilBrown <neilb@suse.com>
Reviewed-on: https://review.whamcloud.com/35980
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Petros Koutoupis <pkoutoupis@cray.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>