Btrfs: Fix super block updates during transaction commit

The super block written during commit was not consistent with the state of
the trees.  This change adds an in-memory copy of the super so that we can
make sure to write out consistent data during a commit.

Signed-off-by: Chris Mason <chris.mason@oracle.com>
diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
index c11ecf5..2e797d5 100644
--- a/fs/btrfs/super.c
+++ b/fs/btrfs/super.c
@@ -144,7 +144,7 @@
 static int btrfs_statfs(struct dentry *dentry, struct kstatfs *buf)
 {
 	struct btrfs_root *root = btrfs_sb(dentry->d_sb);
-	struct btrfs_super_block *disk_super = root->fs_info->disk_super;
+	struct btrfs_super_block *disk_super = &root->fs_info->super_copy;
 
 	buf->f_namelen = BTRFS_NAME_LEN;
 	buf->f_blocks = btrfs_super_total_blocks(disk_super);