Btrfs: return error when we specify wrong start to defrag

We need such a sanity check for wrong start when we defrag a file, otherwise,
even with a wrong start that's larger than file size, we can end up changing
not only inode's force compress flag but also FS's incompat flags.
Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
Reviewed-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>

fs/btrfs/ioctl.c

@@ -1154,8 +1154,11 @@ int btrfs_defrag_file(struct inode *inode, struct file *file,
 	u64 new_align = ~((u64)128 * 1024 - 1);
 	struct page **pages = NULL;
 
-	if (extent_thresh == 0)
-		extent_thresh = 256 * 1024;
+	if (isize == 0)
+		return 0;
+
+	if (range->start >= isize)
+		return -EINVAL;
 
 	if (range->flags & BTRFS_DEFRAG_RANGE_COMPRESS) {
 		if (range->compress_type > BTRFS_COMPRESS_TYPES)
@@ -1164,8 +1167,8 @@ int btrfs_defrag_file(struct inode *inode, struct file *file,
 			compress_type = range->compress_type;
 	}
 
-	if (isize == 0)
-		return 0;
+	if (extent_thresh == 0)
+		extent_thresh = 256 * 1024;
 
 	/*
 	 * if we were not given a file, allocate a readahead