LU-12686 lnet: change ln_mt_waitq to a completion.

LU-12686 lnet: change ln_mt_waitq to a completion.

ln_mt_waitq is only waited on by a call to
  wait_event_interruptible_timeout(..., false, timeout);

As 'false' is never 'true', this will always wait for the full
timeout to expire.  So the waitq is effectively pointless.

To acheive the apparent intent of the waitq, change it to a
completion.  The completion adds a 'done' flag to a waitq so we can
wait until a timeout or until a wakeup is requested.

With this, a longer timeout would could be used, but that is left to
a later patch.

Signed-off-by: Mr NeilBrown <neilb@suse.com>
Change-Id: I10df4b33ee59f579b965e44c022175b8ab007626
Reviewed-on: https://review.whamcloud.com/35874
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Chris Horn <hornc@cray.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Oleg Drokin <green@whamcloud.com>