Revert "nfsd4: remove check_conflicting_opens warning"

This reverts commit 50747dd5 "nfsd4: remove check_conflicting_opens
warning", as a prerequisite for reverting 94415b06, which has a
serious bug.

Cc: stable@vger.kernel.org
Signed-off-by: NJ. Bruce Fields <bfields@redhat.com>
Signed-off-by: NChuck Lever <chuck.lever@oracle.com>

fs/nfsd/nfs4state.c

@@ -4952,6 +4952,7 @@ static int nfsd4_check_conflicting_opens(struct nfs4_client *clp,
 		writes--;
 	if (fp->fi_fds[O_RDWR])
 		writes--;
+	WARN_ON_ONCE(writes < 0);
 	if (writes > 0)
 		return -EAGAIN;
 	spin_lock(&fp->fi_lock);