ipv6: mcast: reduce ipv6_chk_mcast_addr() indentation

Add a label and two gotos to shorten lines by two tabulations,
to ease code review of following patches.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://patch.msgid.link/20241210183352.86530-2-edumazet@google.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>