ZAP: Reduce leaf array and free chunks fragmentation

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 and 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.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Alexander Motin <mav@FreeBSD.org>
Sponsored by: iXsystems, Inc.
Closes #16766