xfrm: Remove useless xfrm_audit struct.

Commit f1370cc4 "xfrm: Remove useless secid field from xfrm_audit." changed
"struct xfrm_audit" to have either
{ audit_get_loginuid(current) / audit_get_sessionid(current) } or
{ INVALID_UID / -1 } pair.

This means that we can represent "struct xfrm_audit" as "bool".
This patch replaces "struct xfrm_audit" argument with "bool".

Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>