ipv6: rpl: Remove redundant multicast tests in ipv6_rpl_srh_rcv().

ipv6_rpl_srh_rcv() checks if ipv6_hdr(skb)->daddr or ohdr->rpl_segaddr[i]
is the multicast address with ipv6_addr_type().

We have the same check for ipv6_hdr(skb)->daddr in ipv6_rthdr_rcv(), so we
need not recheck it in ipv6_rpl_srh_rcv().

Also, we should use ipv6_addr_is_multicast() for ohdr->rpl_segaddr[i]
instead of ipv6_addr_type().

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>