xfrm: Restrict percpu SA attribute to specific netlink message types

Reject the usage of XFRMA_SA_PCPU in xfrm netlink messages when
it's not applicable.

Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>
Tested-by: Antony Antony <antony.antony@secunet.com>
Tested-by: Tobias Brunner <tobias@strongswan.org>