Bridge speculative and prescient prefetchers

[amotin]

There are cases when we need to read large objects sequentially. Prescient prefetcher may be limited by size, trying to prefetch it all at once. Speculative prefetcher can do it nicer for ARC, but needs a ramp-up period at the beginning.

To cover this gap, introduce dmu_prefetch_stream(), priming the speculative prefetcher to either go full speed on first demand request, or even start some prefetch immediately, but continue following the demand later.

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.