nfsd4: zero-length WRITE should succeed

commit fdec6114ee1f0f43b1ad081ad8d46b23ba126d70 upstream.

Zero-length writes are legal; from 5661 section 18.32.3: "If the count
is zero, the WRITE will succeed and return a count of zero subject to
permissions checking".

This check is unnecessary and is causing zero-length reads to return
EINVAL.

Cc: stable@vger.kernel.org
Fixes: 3fd9557a "NFSD: Refactor the generic write vector fill helper"
Cc: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>