nfsd: fix the check for confirmed openowner in nfs4_preprocess_stateid_op

If we find a non-confirmed openowner we jump to exit the function, but do
not set an error value.  Fix this by factoring out a helper to do the
check and properly set the error from nfsd4_validate_stateid.

Cc: stable@vger.kernel.org
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>