Apply the noline attribute to vdev_queue_max_async_writes

This makes it possible to analyze the performance of the new ZFS
write throttle with dtrace

PR: 200316
Submitted by: Lacey Powers <lacey.leanne@gmail.com>
Reviewed by: avg, smh, delphij (no objection)
Approved by: bapt (mentor)
MFC after: 1 month
Sponsored by: ScaleEngine Inc.
Differential Revision: https://reviews.freebsd.org/D3472