btrfs: remove unnecessary variable shadowing in btrfs_invalidatepage()

In btrfs_invalidatepage() we re-declare @tree variable as
btrfs_ordered_inode_tree.

Since it's only used to do the spinlock, we can grab it from inode
directly, and remove the unnecessary declaration completely.

Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>