BACKPORT: 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>

Conflicts: XFRMA_DEV_OFFLOAD not backported with output mark. Removed
from xfrma_policy struct

(cherry picked from commit e719135881f00c01ca400abb8a5dadaf297a24f9)
Signed-off-by: Benedict Wong <benedictwong@google.com>
Bug: 115914689
Change-Id: I7a19fb83ee2153c6c935292d871ad2365359a913