Fix PR18856 by making nfsd.close optional.

Fix PR18856 by making nfsd.close optional.

* tapset/linux/nfsd.stp: Make nfsd.close optional in nfsd.entries (and add
  it optionally to nfsd.return), since the underlying probe point no
  longer exists in kernels 4.2+. Also add nfsd.proc.commit.return to
  nfsd.proc.return (to match nfsd.proc).
* testsuite/buildok/nfsd-detailed.stp: Make nfsd.close tests optional.