xfrm: fix XFRMA_OUTPUT_MARK policy entry

This seems to be an obvious typo, NLA_U32 is type of the attribute, not its
(minimal) length.

Fixes: 077fbac405bf ("net: xfrm: support setting an output mark.")
Signed-off-by: Michal Kubecek <mkubecek@suse.cz>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>