illumos gate

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 the pool is busy with other I/O to preserve
bandwidth.

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
bpobj_iterate() on an empty bpobj. Even though we don't end up
modifying the bpobj it still gets dirtied, causing unneeded writes to the pool.

The fix adds an early bail from bpobj_iterate_impl() if bpobj
is empty to prevent unneeded writes to the pool.