xfrm: remove extract_output indirection from xfrm_state_afinfo

Move this to xfrm_output.c.  This avoids the state->extract_output
indirection.

This patch also removes the duplicated __xfrm6_extract_header helper
added in an earlier patch, we can now use the one from xfrm_inout.h .

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>