Btrfs: fix crash in log replay with qgroups enabled

When replaying a log tree with qgroups enabled, tree_mod_log_rewind does a
sanity-check of the number of items against the maximum possible number.
It calculates that number with the nodesize of fs_root. Unfortunately
fs_root is not yet set at this stage. So instead use the nodesize from
tree_root, which is already initialized.
Signed-off-by: Arne Jansen <sensille@gmx.net>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>

fs/btrfs/ctree.c

@@ -1222,7 +1222,7 @@ tree_mod_log_rewind(struct btrfs_fs_info *fs_info, struct extent_buffer *eb,
 
 	__tree_mod_log_rewind(eb_rewin, time_seq, tm);
 	WARN_ON(btrfs_header_nritems(eb_rewin) >
-		BTRFS_NODEPTRS_PER_BLOCK(fs_info->fs_root));
+		BTRFS_NODEPTRS_PER_BLOCK(fs_info->tree_root));
 
 	return eb_rewin;
 }