Btrfs: fix wrong max_to_defrag in btrfs_defrag_file()

It's off-by-one, and thus we may skip the last page while defragmenting.

An example case:

  # create /mnt/file with 2 4K file extents
  # btrfs fi defrag /mnt/file
  # sync
  # filefrag /mnt/file
  /mnt/file: 2 extents found

So it's not defragmented.
Signed-off-by: NLi Zefan <lizf@cn.fujitsu.com>

fs/btrfs/ioctl.c

@@ -1052,7 +1052,7 @@ int btrfs_defrag_file(struct inode *inode, struct file *file,
 		i = range->start >> PAGE_CACHE_SHIFT;
 	}
 	if (!max_to_defrag)
-		max_to_defrag = last_index - 1;
+		max_to_defrag = last_index;
 
 	while (i <= last_index && defrag_count < max_to_defrag) {
 		/*