xfrm: remove gso_segment indirection from xfrm_mode

xfrm: remove gso_segment indirection from xfrm_mode

These functions are small and we only have versions for tunnel
and transport mode for ipv4 and ipv6 respectively.

Just place the 'transport or tunnel' conditional in the protocol
specific function instead of using an indirection.

Before:
    3226       12       0     3238   net/ipv4/esp4_offload.o
    7004      492       0     7496   net/ipv4/ip_vti.o
    3339       12       0     3351   net/ipv6/esp6_offload.o
   11294      460       0    11754   net/ipv6/ip6_vti.o
    1180       72       0     1252   net/ipv4/xfrm4_mode_beet.o
     428       48       0      476   net/ipv4/xfrm4_mode_transport.o
    1271       48       0     1319   net/ipv4/xfrm4_mode_tunnel.o
    1083       60       0     1143   net/ipv6/xfrm6_mode_beet.o
     172       48       0      220   net/ipv6/xfrm6_mode_ro.o
     429       48       0      477   net/ipv6/xfrm6_mode_transport.o
    1164       48       0     1212   net/ipv6/xfrm6_mode_tunnel.o
15730428  6937008 4046908 26714344   vmlinux

After:
    3461       12       0     3473   net/ipv4/esp4_offload.o
    7000      492       0     7492   net/ipv4/ip_vti.o
    3574       12       0     3586   net/ipv6/esp6_offload.o
   11295      460       0    11755   net/ipv6/ip6_vti.o
    1180       64       0     1244   net/ipv4/xfrm4_mode_beet.o
     171       40       0      211   net/ipv4/xfrm4_mode_transport.o
    1163       40       0     1203   net/ipv4/xfrm4_mode_tunnel.o
    1083       52       0     1135   net/ipv6/xfrm6_mode_beet.o
     172       40       0      212   net/ipv6/xfrm6_mode_ro.o
     172       40       0      212   net/ipv6/xfrm6_mode_transport.o
    1056       40       0     1096   net/ipv6/xfrm6_mode_tunnel.o
15730424  6937008 4046908 26714340   vmlinux

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