[IPSEC] Fix xfrm to pfkey SA state conversion

This patch adjusts the SA state conversion in af_key such that
XFRM_STATE_ERROR/XFRM_STATE_DEAD will be converted to SADB_STATE_DEAD
instead of SADB_STATE_DYING.

According to RFC 2367, SADB_STATE_DYING SAs can be turned into
mature ones through updating their lifetime settings.  Since SAs
which are in the states XFRM_STATE_ERROR/XFRM_STATE_DEAD cannot
be resurrected, this value is unsuitable.

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