ZAP: Reduce leaf array and free chunks fragmentation

[amotin]

Previous implementation of zap_leaf_array_free() put chunks on the free list in reverse order. Also zap_leaf_transfer_entry() and zap_entry_remove() were freeing name and value arrays in reverse order. Together this created a mess in the free list, making following allocations much more fragmented than necessary.

This patch re-implements zap_leaf_array_free() to keep existing chunks order, and implements non-destructive zap_leaf_array_copy() to be used in zap_leaf_transfer_entry() to allow properly ordered freeing name and value arrays there, as in zap_entry_remove().

With this change test of some writes and deletes shows percent of non-contiguous chunks in DDT reducing from 61% and 47% to 0% and 17% for arrays and frees respectively. Sure some explicit sorting could do even better, especially for ZAPs with variable-size arrays, but it would also cost much more, while this should be very cheap.

Another improvement is that previously zap_entry_update() for multi-chunk values always reverted chunk order, changing the leaf block even if nothing has actually changed. I don't know if we can benefit from the block not changing via nop-write or something, but it should not harm to be more predictable.

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.