xfrm: Remove inner/outer modes from output path

The inner/outer modes were added to abstract out common code that
were once duplicated between IPv4 and IPv6.  As time went on the
abstractions have been removed and we are now left with empty
shells that only contain duplicate information.  These can be
removed one-by-one as the same information is already present
elsewhere in the xfrm_state object.

Just like the input-side, removing this from the output code
makes it possible to use transport-mode SAs underneath an
inter-family tunnel mode SA.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>