nfsd4: fix share mode permissions

NFSv4 opens may function as locks denying other NFSv4 users the rights
to open a file.

We're requiring a user to have write permissions before they can deny
write.  We're *not* requiring a user to have write permissions to deny
read, which is if anything a more drastic denial.

What was intended was to require write permissions for DENY_READ.
Signed-off-by: NJ. Bruce Fields <bfields@citi.umich.edu>

fs/nfsd/nfs4proc.c

@@ -171,7 +171,7 @@ do_open_permission(struct svc_rqst *rqstp, struct svc_fh *current_fh, struct nfs
 		accmode |= NFSD_MAY_READ;
 	if (open->op_share_access & NFS4_SHARE_ACCESS_WRITE)
 		accmode |= (NFSD_MAY_WRITE | NFSD_MAY_TRUNC);
-	if (open->op_share_deny & NFS4_SHARE_DENY_WRITE)
+	if (open->op_share_deny & NFS4_SHARE_DENY_READ)
 		accmode |= NFSD_MAY_WRITE;
 
 	status = fh_verify(rqstp, current_fh, S_IFREG, accmode);