nfsd: fix the check for confirmed openowner in nfs4_preprocess_stateid_op

commit ebe9cb3bb13e7b9b281969cd279ce70834f7500f upstream.

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.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>