Btrfs: check file pointer in btrfs_sync_file

fsync can be called by NFS with a null file pointer, and btrfs was
oopsing in this case.

Signed-off-by: Chris Mason <chris.mason@oracle.com>

fs/btrfs/file.c

@@ -1222,7 +1222,7 @@ int btrfs_sync_file(struct file *file, struct dentry *dentry, int datasync)
 	/*
 	 * ok we haven't committed the transaction yet, lets do a commit
 	 */
-	if (file->private_data)
+	if (file && file->private_data)
 		btrfs_ioctl_trans_end(file);
 
 	trans = btrfs_start_transaction(root, 1);
@@ -1231,7 +1231,7 @@ int btrfs_sync_file(struct file *file, struct dentry *dentry, int datasync)
 		goto out;
 	}
 
-	ret = btrfs_log_dentry_safe(trans, root, file->f_dentry);
+	ret = btrfs_log_dentry_safe(trans, root, dentry);
 	if (ret < 0)
 		goto out;
 
@@ -1245,15 +1245,15 @@ int btrfs_sync_file(struct file *file, struct dentry *dentry, int datasync)
 	 * file again, but that will end up using the synchronization
 	 * inside btrfs_sync_log to keep things safe.
 	 */
-	mutex_unlock(&file->f_dentry->d_inode->i_mutex);
+	mutex_unlock(&dentry->d_inode->i_mutex);
 
 	if (ret > 0) {
 		ret = btrfs_commit_transaction(trans, root);
 	} else {
 		btrfs_sync_log(trans, root);
 		ret = btrfs_end_transaction(trans, root);
 	}
-	mutex_lock(&file->f_dentry->d_inode->i_mutex);
+	mutex_lock(&dentry->d_inode->i_mutex);
 out:
 	return ret > 0 ? EIO : ret;
 }