nat: fix accidental o2i deletion/reuse

nat: fix accidental o2i deletion/reuse

Nat session is allocated before the port allocation. During port allocation
candidate address+port are set to o2i 6-tuple and tested against the flow hash.
If insertion fails, the port is busy and rejected. When all N attempts are
unsuccessful, "out-of-ports" error is recorded and the session is to be
deleted.

During session deletion o2i and i2o tuples are deleted from the flow hash.
In case of "out-of-ports" i2o tuple is not valid, however o2i is and it refers
to **some other** session that's known to be allocated.

By backing match tuple up session should be invalidated well enough not to
collide with any valid one.

Type: fix
Signed-off-by: Dmitry Valter <d-valter@yandex-team.ru>
Change-Id: Id30be6f26ecce7a5a63135fb971bb65ce318af82