datapath: Do not create NAT entry for egress ICMP ECHO_REPLY
datapath: Do not create NAT entry for egress ICMP ECHO_REPLY

Previously, when ICMP ECHO was sent from outside to a host managed by
Cilium, the handling of the reply to it (ICMP ECHO_REPLY) used to create
the following entries:

CT  |    src     |     dst   | dir |
    +------------+-----------+-----+
    | outside:0  |  host:ID  | OUT |

NAT |    src     |    dst    | dir |
    +------------+-----------+-----+
    | host:0     | outside:ID| OUT | <-- ICMP ECHO_REPLY
    +------------+-----------+-----+
    | outside:ID |   host:ID | IN  | <-- ICMP ECHO

The NAT IN entry was useful only to avoid pod->outside to be SNAT-ed
with the same ID, but this is no longer the case after the
"datapath: Fix unintended SNAT of ICMP ECHO" commit.

Also, this removes the problematic CT GC case in which for such a CT
entry a corresponding NAT OUT entry with the existing GC logic could not
be found.

Signed-off-by: Martynas Pumputis <6b0d31c0d563223024da45691584643ac78c96e8@lambda.lt>

tgraf/cilium,cilium/cilium,michi-covalent/cilium,tklauser/cilium,tklauser/cilium,cilium/cilium,tgraf/cilium,cilium/cilium,cilium/cilium,tgraf/cilium,michi-covalent/cilium,tklauser/cilium,tklauser/cilium,cilium/cilium,tgraf/cilium,tklauser/cilium,tgraf/cilium,michi-covalent/cilium,michi-covalent/cilium,michi-covalent/cilium,tgraf/cilium