ikev2: accept rekey request for IKE SA

ikev2: accept rekey request for IKE SA

RFC 7296 describes the way to rekey IKE SAs: to rekey an IKE SA,
establish a new equivalent IKE SA with the peer to whom the old
IKE SA is shared using a CREATE_CHILD_SA within the existing IKE
SA.  An IKE SA so created inherits all of the original IKE SA's
Child SAs, and the new IKE SA is used for all control messages
needed to maintain those Child SAs.

Type: improvement
Signed-off-by: Atzm Watanabe <atzmism@gmail.com>
Change-Id: Icdf43b67c38bf183913a28a08a85236ba16343af