[XFRM]: Fix OOPSes in xfrm_audit_log().

Make sure that this function is called correctly, and
add BUG() checking to ensure the arguments are sane.

Based upon a patch by Joy Latten.
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>