LU-12718 obdclass: Allow read-ahead for write requests

LU-12718 obdclass: Allow read-ahead for write requests

cl_io_read_ahead asserts that read-ahead can only happen
due to CIT_READ or CIT_FAULT requests.
Since LU-9618, we expect CIT_WRITE requests to also
sometimes trigger read-ahead.
So the LINVRNT() needs to be extended to acknowledge
that.

Signed-off-by: Mr NeilBrown <neilb@suse.com>
Change-Id: I7aa1efb4fc8bb6f8474596a6194fc39f484d7ac7
Reviewed-on: https://review.whamcloud.com/36000
Reviewed-by: Shilong Wang <wshilong@ddn.com>
Reviewed-by: Patrick Farrell <pfarrell@whamcloud.com>
Reviewed-by: Neil Brown <neilb@suse.de>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>