dm-raid: fix a race condition in request handling

raid_map calls pers->make_request, which missed the suspend check. Fix it with
the new md_handle_request API.

Fix: cc27b0c7(md: fix deadlock between mddev_suspend() and md_write_start())
Cc: Heinz Mauelshagen <heinzm@redhat.com>
Cc: Mike Snitzer <snitzer@redhat.com>
Cc: stable@vger.kernel.org
Reviewed-by: NeilBrown <neilb@suse.com>
Signed-off-by: Shaohua Li <shli@fb.com>