xfrm: clone XFRMA_SEC_CTX in xfrm_do_migrate

XFRMA_SEC_CTX was not cloned from the old to the new.
Migrate this attribute during XFRMA_MSG_MIGRATE

v1->v2:
 - return -ENOMEM on error
v2->v3:
 - fix return type to int

Fixes: 80c9abaa ("[XFRM]: Extension for dynamic update of endpoint address(es)")
Signed-off-by: Antony Antony <antony.antony@secunet.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>