ARC: Increase parallel eviction batching

[amotin]

Before parallel eviction implementation zfs_arc_evict_batch_limit caused loop exits after evicting 10 headers. The cost of it is not big and well motivated. Now though taskq task exit after the same 10 headers is much more expensive. To cover the context switch overhead of taskq introduce another level of batching, controlled by zfs_arc_evict_batches_limit tunable, used only for parallel eviction.

How Has This Been Tested?

My tests including 36 parallel read threads with 4KB recordsize, that shown 1.4GB/s (~460K blocks/s) before with heavy arc_evict_lock contention, now show 6.5GB/s (~1.6M blocks/s) without arc_evict_lock contention. Attempts to further increase the new batch size beyond the 5 iterations shows diminishing return.

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [x] 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:

• [x] 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.
• [x] All commit messages are properly formatted and contain Signed-off-by.