LU-10467 ptlrpc: convert use of l_wait_event_exclusive_head()

LU-10467 ptlrpc: convert use of l_wait_event_exclusive_head()

Only one place uses l_wait_event_exclusive_head().
It uses an on_timeout function that returns non-zero, so
the wait aborts after timeout.

Change this to wait_event_idle_exclusive_lifo_timeout(),
and if it times out, perform the same action as the
on_timeout handler - a simple assignment.

Signed-off-by: Mr NeilBrown <neilb@suse.com>
Change-Id: I11bee6aa1eceb6564fb72e41528f2f6a80b0d207
Reviewed-on: https://review.whamcloud.com/35986
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Petros Koutoupis <pkoutoupis@cray.com>
Reviewed-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>