icmp: Fix wrong icmp header cast

This was introduced by efa391c40c414e7071b2041f1888180deb1806b9
icmp: Don't convert saved ip header's length into host byte order

No kernel code depends on this and it actually whacks the icmp header
passed to userland through SOCK_RAW socket.  ICMP redirect processing
does not need fix (it does not do the convertion).