This issue tracks the port of scrub pause from ZoL: https://github.com/zfsonlinux/zfs/pull/6167
Currently, there is no way to pause a scrub. Pausing may be useful when

Description

This patch adds the ability to pause and resume scrubbing.  This is achieved
by maintaining a persistent on-disk scrub state.  While the state is 'paused'
we do not scrub any more blocks.  We do however perform regular scan
housekeeping such as freeing async destroyed and deadlist blocks while paused.

If you're testing this change, you probably want to include the patch from #6164

Motivation and Context

Scrub pausing can be an I/O intensive operation and people have been asking
for the ability to pause a scrub for a while. This allows one to preserve scrub
progress while freeing up bandwidth for other I/O.

How Has This Been Tested?

Unit testing and zfs-tests.  to the pool.  This patch will also include the
patch from https://github.com/zfsonlinux/zfs/ pull/6164 In certain cases
(dsl_scan_sync() is one), we may end up calling