nfsd4: don't treat readlink like a zero-copy operation

There's no advantage to this zero-copy-style readlink encoding, and it
unnecessarily limits the kinds of compounds we can handle.  (In practice
I can't see why a client would want e.g. multiple readlink calls in a
comound, but it's probably a spec violation for us not to handle it.)

Signed-off-by: J. Bruce Fields <bfields@redhat.com>