xfrm: move mark and oif flowi decode into common code

flowi4_oif/flowi6_oif and mark are aliased to flowi_common field, i.e.
all can be used interchangeably.

Instead of duplicating place this in common code.
No functional changes intended.

Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>