zfs list burning lots of time in dodefault() via dsl_prop_*
Flamegraphing "zfs list" commands suggests that there is unnecessary time being burnt in dodefault() – see https://us-east.manta.joyent.com/arekinath/public/zfs-flame3.html
dodefault() takes a string argument, the name of a ZFS property, then looks it up to get a zfs_prop_t using zfs_name_to_prop. However, in most of the places dodefault() is called, the zfs_prop_t is already known in the function calling. We can just pass it through and avoid wasting time here.