ipv6: raw: Deduct extension header length in rawv6_push_pending_frames

The total cork length created by ip6_append_data includes extension
headers, so we must exclude them when comparing them against the
IPV6_CHECKSUM offset which does not include extension headers.

Reported-by: Kyle Zeng <zengyhkyle@gmail.com>
Fixes: 357b40a1 ("[IPV6]: IPV6_CHECKSUM socket option can corrupt kernel memory")
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>