net: ipv6: exthdrs: get rid of ipv6_skb_net()

Get rid of ipv6_skb_net() which is only used in ipv6_hop_ioam().

Signed-off-by: Justin Iurman <justin.iurman@uliege.be>
Link: https://lore.kernel.org/r/20240308185343.39272-1-justin.iurman@uliege.be
Signed-off-by: Jakub Kicinski <kuba@kernel.org>