fs/9p: Fix atomic_open

Return EEXISTS if requested file already exists, without this patch open
call will always succeed even if the file exists and user specified
O_CREAT|O_EXCL.

Following test code can be used to verify this patch. Without this patch
executing following test code on 9p mount will result in printing 'test case
failed' always.

main()
{
        int fd;

        /* first create the file */
        fd = open("./file", O_CREAT|O_WRONLY);
        if (fd < 0) {
                perror("open");
                return -1;
        }
        close(fd);

        /* Now opening same file with O_CREAT|O_EXCL should fail */
        fd = open("./file", O_CREAT|O_EXCL);
        if (fd < 0 && errno == EEXIST)
	        printf("test case pass\n");
        else
	        printf("test case failed\n");
        close(fd);
        return 0;
}

Signed-off-by: M. Mohan Kumar <mohan@in.ibm.com>
Signed-off-by: Eric Van Hensbergen <ericvh@gmail.com>

fs/9p/vfs_inode_dotl.c

@@ -267,8 +267,14 @@ v9fs_vfs_atomic_open_dotl(struct inode *dir, struct dentry *dentry,
 	}
 
 	/* Only creates */
-	if (!(flags & O_CREAT) || dentry->d_inode)
-		return finish_no_open(file, res);
+	if (!(flags & O_CREAT))
+		return	finish_no_open(file, res);
+	else if (dentry->d_inode) {
+		if ((flags & (O_CREAT | O_EXCL)) == (O_CREAT | O_EXCL))
+			return -EEXIST;
+		else
+			return finish_no_open(file, res);
+	}
 
 	v9ses = v9fs_inode2v9ses(dir);