Update cache file when setting compatibility property

[behlendorf]

Motivation and Context

Issue #12261 observed the compatibility does not persist when
importing the pool using a cache file.

Description

Unlike most other properties the 'compatibility' property is stored
in the pool config object and not the DMU_OT_POOL_PROPS object.

This had the advantage that the compatibility information is available
without needing to fully import the pool (it can be read with zdb).
However, this means we need to make sure to update both the copy of
the config in the MOS and the cache file. This wasn't being done.

This commit adds a call to spa_async_request() to ensure the copy of
the config in the cache file gets updated as well as the one stored
in the pool. This same change is made for the 'comment' property
which suffers from the same inconsistency.

How Has This Been Tested?

Added a new zpool_create_features_009_pos.ksh test case which
verifies the property is stored where it should be and is correctly
set when importing from a cache file.

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)
• Library ABI change (libzfs, libzfs_core, libnvpair, libuutil and libzfsbootenv)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the OpenZFS code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• I have run the ZFS Test Suite with this change applied.
• All commit messages are properly formatted and contain Signed-off-by.