MFV r275546:

Reduce scrub activities when system there is enough dirty data, namely when
dirty data is more than zfs_vdev_async_write_active_min_dirty_percent (once
we start to increase the number of concurrent async writes).

While there also correct rounding error which would make scrub end up
pausing for (zfs_txg_timeout + 1) seconds instead of the desired
zfs_txg_timeout seconds.

Illumos issue:

5351 scrub goes for an extra second each txg
5352 scrub should pause when there is some dirty data

MFC after: 2 weeks