btrfs: use min() to replace open-code in btrfs_invalidatepage()

In btrfs_invalidatepage() we introduce a temporary variable, new_len, to
update ordered->truncated_len.  But we can use min() to replace it
completely and no need for the variable.

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>