Improve caching for dbuf prefetches

[amotin]

To avoid read errors with transaction open dmu_tx_check_ioerr() is used to read everything required in advance. But there seems to be a chance for the buffer to be evicted from dbuf cache in between, which results in immediate eviction from ARC, which may require additional disk read later, in a place where error handling is problematic.

To partially workaround this introduce a new flag DMU_IS_PREFETCH, relayed to ARC as ARC_FLAG_PREFETCH | ARC_FLAG_PRESCIENT_PREFETCH, making ARC delay eviction by at least several seconds, or till the actual read inside the transaction, that will promote it to demand access.

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)
• Quality assurance (non-breaking change which makes the code more robust against bugs)
• 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.