ipv6_rthdr_rcv() pulls these data

  - Segment Routing Header : 8
  - Hdr Ext Len            : skb_transport_header(skb)[1] << 3

needed by ipv6_srh_rcv(), so pskb_pull() in ipv6_srh_rcv() never
fails and can be replaced with skb_pull().
Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>