xfrm: remove output indirection from xfrm_mode

Same is input indirection.  Only exception: we need to export
xfrm_outer_mode_output for pktgen.

Increases size of vmlinux by about 163 byte:
Before:
   text    data     bss     dec      filename
15730208  6936948 4046908 26714064   vmlinux

After:
15730311  6937008 4046908 26714227   vmlinux

xfrm_inner_extract_output has no more external callers, make it static.

v2: add IS_ENABLED(IPV6) guard in xfrm6_prepare_output
    add two missing breaks in xfrm_outer_mode_output (Sabrina Dubroca)
    add WARN_ON_ONCE for 'call AF_INET6 related output function, but
    CONFIG_IPV6=n' case.
    make xfrm_inner_extract_output static

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