net: ip: igmp: Fix wrong header length

The header length of the net ip packet was calculated using only the
net_pkt_ip_hdr_len function. The correct header length should be
calculated by adding net_pkt_ip_hdr_len and net_pkt_ipv4_opts_len. This
resulted in an incorrect IGMP version type in case of IGMPv2 message
(when IGMPv3 was enabled). The IGMP message was not parsed correctly and
therefore dropped.

Signed-off-by: Ibe Van de Veire <ibe.vandeveire@basalte.be>