The macOS NFS client sends SETATTR after SYMLINK, which arrives as
Chmod/Chown on the link path. These opened the target with vfs.Open,
which follows symlinks - a freshly created symlink usually dangles, so
the open failed with ENOENT, surfaced to the client as NFS3ERR_IO even
though the link was created.
Add path-based VFS.Chmod and VFS.Chown mirroring VFS.Chtimes. They do
not follow symlinks (lstat semantics, matching VFS.Stat) and return
ENOSYS when the node exists, since the VFS stores neither permissions
nor ownership; serve nfs calls them and masks ENOSYS as before.
Fixes#9627
