ipsec: Allow SAs with NULL auth &crypto on IPSec interface

ipsec: Allow SAs with NULL auth &crypto on IPSec interface

Type: improvement

on the dedicated IPSec interface, the SA describes the peer, so it is
not possible to forward to a peer for which there is no SA. Therefore if
an SA is added with NULL auth and integ then this explicitly states that
this is what the peer desires.
on the contrary on the IP-IP/GRE interface, in the absence of protection
and an SA, then the traffic is sent in the clear. So adding NULL
auth/crypto iSA is a means to describe that the peer should not be sent
traffic.

Signed-off-by: Neale Ranns <nranns@cisco.com>
Change-Id: I7ad2d466cc74eb7ff8c4c84e0d7897d06e2fcf86