Btrfs: do not defrag a file partially

xfstests 218 complains that btrfs defrags a file partially:
 After: 1
 Write backwards sync, but contiguous - should defrag to 1 extent
 Before: 10
-After: 1
+After: 2

To fix this, we need to set max_to_defrag count properly.

Signed-off-by: Liu Bo <liubo2009@cn.fujitsu.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>
diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
index 6834be4..0b06a5c 100644
--- a/fs/btrfs/ioctl.c
+++ b/fs/btrfs/ioctl.c
@@ -1066,7 +1066,7 @@
 		i = range->start >> PAGE_CACHE_SHIFT;
 	}
 	if (!max_to_defrag)
-		max_to_defrag = last_index;
+		max_to_defrag = last_index + 1;
 
 	/*
 	 * make writeback starts from i, so the defrag range can be