The "FTS_NOSTAT" option can avoid a lot of calls to stat(2) if it knows that a
directory could not possibly have subdirectories. This is decided by looking at
the link count: a subdirectory would increment its parent's link count by
virtue of its own ".." entry. This assumption only holds for UFS-like
filesystems that implement links and directories this way, so we must punt for
others.
It looks like ZFS is a UFS-like file system, as the above also holds for ZFS.
Add ZFS to the list of file systems that allow for such optimization.
MFC after: 1 month
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