net: icmpv4: drop requests on NET_OFFLOAD interfaces

NET_OFFLOAD interfaces do not handle ICMP request. Request to send ICMP
packet resulted in NULL pointer dereference in net_if_tx() later
on. Prevent that by detecting NET_OFFLOAD interfaces early in icmpv4
module.

Signed-off-by: Marcin Niestroj <m.niestroj@grinn-global.com>