xfrm: fix XFRMA_MARK extraction in xfrm_mark_get

Determine the size of the xfrm_mark struct, not of its pointer.

Signed-off-by: Andreas Steffen <andreas.steffen@strongswan.org>
Acked-by: Jamal Hadi Salim <hadi@cyberus.ca>
Signed-off-by: David S. Miller <davem@davemloft.net>