Speed up the (common) case where no nodes in an experiment are in the MFS.
On nfree, we call nfsmfs_setup will all nodes in the experiment even though it is unlikely that any of them are in the MFS. Even so, we were unconditionally doing a "zfs share -a" to make sure the exports table was up to date. This hurt hard in our environment where we have a zillion users and oodles of nodes (these are very precise thresholds!) Also, use "mount" to get the list of mounted NFS MFS filesystems rather than "zfs list" which likewise can be quite painful when there are a gazillion (1 gazillion > 1 zillion) ZFS filesystems.
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