MFC r320352: zfs: port vdev_file part of illumos change 3306

3306 zdb should be able to issue reads in parallel
illumos/illumos-gate/31d7e8fa33fae995f558673adb22641b5aa8b6e1
https://www.illumos.org/issues/3306

The upstream change was made before we started to import upstream commits
individually. It was imported into the illumos vendor area as r242733.
That commit was MFV-ed in r260138, but as the commit message says
vdev_file.c was left intact.

This commit actually implements the parallel I/O for vdev_file using a
taskqueue with multiple thread. This implementation does not depend on
the illumos or FreeBSD bio interface at all, but uses zio_t to pass
around all the relevent data. So, the code looks a bit different from
the upstream.

This commit also incorporates ZoL commit
zfsonlinux/zfs/bc25c9325b0e5ced897b9820dad239539d561ec9 that fixed
https://github.com/zfsonlinux/zfs/issues/2270
We need to use a dedicated taskqueue for exactly the same reason as ZoL
as we do not implement TASKQ_DYNAMIC.