ipv6: align code with context

The Tab key is used three times, causing the code block to
be out of alignment with the context.
Signed-off-by: Rocco Yue <rocco.yue@mediatek.com>
Link: https://lore.kernel.org/r/20210530113811.8817-1-rocco.yue@mediatek.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>