net: rpl: fix rpl header size calculation

This patch fixes a missing 8 byte for the header size calculation. The
ipv6_rpl_srh_size() is used to check a skb_pull() on skb->data which
points to skb_transport_header(). Currently we only check on the
calculated addresses fields using CmprI and CmprE fields, see:

https://www.rfc-editor.org/rfc/rfc6554#section-3

there is however a missing 8 byte inside the calculation which stands
for the fields before the addresses field. Those 8 bytes are represented
by sizeof(struct ipv6_rpl_sr_hdr) expression.

Fixes: 8610c7c6 ("net: ipv6: add support for rpl sr exthdr")
Signed-off-by: Alexander Aring <aahringo@redhat.com>
Reported-by: maxpl0it <maxpl0it@protonmail.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/rpl.c

@@ -32,7 +32,8 @@ static void *ipv6_rpl_segdata_pos(const struct ipv6_rpl_sr_hdr *hdr, int i)
 size_t ipv6_rpl_srh_size(unsigned char n, unsigned char cmpri,
 			 unsigned char cmpre)
 {
-	return (n * IPV6_PFXTAIL_LEN(cmpri)) + IPV6_PFXTAIL_LEN(cmpre);
+	return sizeof(struct ipv6_rpl_sr_hdr) + (n * IPV6_PFXTAIL_LEN(cmpri)) +
+		IPV6_PFXTAIL_LEN(cmpre);
 }
 
 void ipv6_rpl_srh_decompress(struct ipv6_rpl_sr_hdr *outhdr,