Add additional sanity check when resuming a pool

[behlendorf]

Description

A pool may only be resumed when the txg in the "best" uberblock
found on-disk matches the in-core last synced txg. This is done
to verify that the pool was not modified by another node or process
while it was suspended. If this were to happen the result would
be a corrupted pool.

Since a suspended pool may no longer always be resumable it was
necessary to extend the 'zpool export -F` command to allow a
suspended pool to be exported. This was accomplished by leveraging
the existing spa freeze functionality. During export if '-F' is
given and the pool is suspended the pool will be frozen at the last
synced txg and all in-core dirty data will be discarded. This
allows for the pool to be safely exported without having to reboot
the system.

In order to test this functionality the broken 'ztest -E' option,
which allows for ztest to use an existing pool, was fixed. The
code needed for this was copied over from zdb. ztest is used to
modify the test pool from user space while the kernel has the pool
imported and suspended.

This commit partially addresses issues #4003, #2023, #2878, #3256
by allowing a suspended pool to be exported, 'zpool export -F'.
There may still be cases where a reference on the pool, such as
a filesystem which cannot be unmounted, will prevent the pool
from being exported.

Motivation and Context

Related to multi-modifier protection, PR #6073.

How Has This Been Tested?

Added test case zpool_clear-resume.ksh.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the ZFS on Linux code style requirements.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.
• All commit messages are properly formatted and contain Signed-off-by.
• Change has been approved by a ZFS on Linux member.

[mention-bot]

@behlendorf, thanks for your PR! By analyzing the history of the files in this pull request, we identified @FransUrbo, @dswartz and @ahrens to be potential reviewers.

[bunder2015]

Had a question in IRC, was this patch merged? Is it still needed after MMP was merged?

[behlendorf]

This patch isn't needed for MMP it only provides an additional level of safely when resuming a suspended pool. It still needs some work before it can be merged, which we plan to do.