NFSD: Update the NFSv2 READLINK argument decoder to use struct xdr_stream

If the code that sets up the sink buffer for nfsd_readlink() is
moved adjacent to the nfsd_readlink() call site that uses it, then
the only argument is a file handle, and the fhandle decoder can be
used instead.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>