Reduce async_destroy_001_pos memory requirements

[behlendorf]

Description

The number of blocks which can be freed per TXG is controlled by the zfs_free_max_blocks module option (defaults to 100,000). Both speed up this test case and reduce the memory requirements by only creating 4 TXGs worth of blocks to be freed.

Motivation and Context

Resolve occasional OOM events during automated testing due to the relatively little memory allocated to the VMs. Additionally, enable these test cases for the 32-bit and kmemleak builders.

How Has This Been Tested?

Locally tested.

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.