stand: Limit array passed to fzap_leaf_array to arrays size

[imp]

Summary

In zap_leaf_lookup, bound the array size passed to fzap_leaf_array to
the smaller of what's in the leaf node and the passed in array size. We
do not need to read more than the number of ints in the entry, so bound
what we pass to the fzap_leaf_read to this number. This is currently a
nop, but the dynamic system attributes code will need it.

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