xfrm: remove output2 indirection from xfrm_mode

similar to previous patch: no external module dependencies,
so we can avoid the indirection by placing this in the core.

This change removes the last indirection from xfrm_mode and the
xfrm4|6_mode_{beet,tunnel}.c modules contain (almost) no code anymore.

Before:
   text    data     bss     dec     hex filename
   3957     136       0    4093     ffd net/xfrm/xfrm_output.o
    587      44       0     631     277 net/ipv4/xfrm4_mode_beet.o
    649      32       0     681     2a9 net/ipv4/xfrm4_mode_tunnel.o
    625      44       0     669     29d net/ipv6/xfrm6_mode_beet.o
    599      32       0     631     277 net/ipv6/xfrm6_mode_tunnel.o
After:
   text    data     bss     dec     hex filename
   5359     184       0    5543    15a7 net/xfrm/xfrm_output.o
    171      24       0     195      c3 net/ipv4/xfrm4_mode_beet.o
    171      24       0     195      c3 net/ipv4/xfrm4_mode_tunnel.o
    172      24       0     196      c4 net/ipv6/xfrm6_mode_beet.o
    172      24       0     196      c4 net/ipv6/xfrm6_mode_tunnel.o

v2: fold the *encap_add functions into xfrm*_prepare_output
    preserve (move) output2 comment (Sabrina)
    use x->outer_mode->encap, not inner
    fix a build breakage on ppc (kbuild robot)

Signed-off-by: Florian Westphal <fw@strlen.de>
Reviewed-by: Sabrina Dubroca <sd@queasysnail.net>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>