[IPSEC] Use XFRM_MSG_* instead of XFRM_SAP_*

This patch removes XFRM_SAP_* and converts them over to XFRM_MSG_*.
The netlink interface is meant to map directly onto the underlying
xfrm subsystem.  Therefore rather than using a new independent
representation for the events we can simply use the existing ones
from xfrm_user.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>