LU-10467 ldlm: use wait_event_idle() instead of l_wait_event

LU-10467 ldlm: use wait_event_idle() instead of l_wait_event

This l_wait_event() is equivalent to wait_event_idle() which is now
supported in lustre.  So switch over to it.

Signed-off-by: Mr NeilBrown <neilb@suse.de>
Change-Id: If1ee81a0d562516534665d049fb24c1f39b59b95
Reviewed-on: https://review.whamcloud.com/37609
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>