Btrfs: fix freeze vs auto defrag

If we freeze the fs, the auto defragment should not run. Fix it.

Signed-off-by: Miao Xie <miaox@cn.fujitsu.com>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>

fs/btrfs/file.c

diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c
index 3c6f747..d415a05 100644
--- a/fs/btrfs/file.c
+++ b/fs/btrfs/file.c
@@ -318,8 +318,11 @@
 	memset(&range, 0, sizeof(range));
 	range.len = (u64)-1;
 	range.start = defrag->last_offset;
+
+	sb_start_write(fs_info->sb);
 	num_defrag = btrfs_defrag_file(inode, NULL, &range, defrag->transid,
 				       BTRFS_DEFRAG_BATCH);
+	sb_end_write(fs_info->sb);
 	/*
 	 * if we filled the whole defrag batch, there
 	 * must be more work to do.  Queue this defrag