xfrm: remove xmit indirection from xfrm_mode

There are only two versions (tunnel and transport). The ip/ipv6 versions
are only differ in sizeof(iphdr) vs ipv6hdr.

Place this in the core and use x->outer_mode->encap type to call the
correct adjustment helper.

Before:
   text   data    bss     dec      filename
15730311  6937008 4046908 26714227 vmlinux

After:
15730428  6937008 4046908 26714344 vmlinux

(about 117 byte increase)

v2: use family from x->outer_mode, not inner

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