net: icmp: Remove NET_ICMP_HDR() macro and direct access to net_buf

Remove NET_ICMP_HDR() macro as we cannot safely access ICMP header
via it if the network packet header spans over multiple net_buf
fragments.

Jira: ZEP-2306

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>