xfrm: remove input2 indirection from xfrm_mode

No external dependencies on any module, place this in the core.
Increase is about 1800 byte for xfrm_input.o.

The beet helpers get added to internal header, as they can be reused
from xfrm_output.c in the next patch (kernel contains several
copies of them in the xfrm{4,6}_mode_beet.c files).

Before:
   text    data     bss     dec filename
   5578     176    2364    8118 net/xfrm/xfrm_input.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

After:
   text    data     bss     dec filename
   7373     200    2364    9937 net/xfrm/xfrm_input.o
    587      44       0     631 net/ipv4/xfrm4_mode_beet.o
    171      32       0     203 net/ipv4/xfrm4_mode_transport.o
    649      32       0     681 net/ipv4/xfrm4_mode_tunnel.o
    625      44       0     669 net/ipv6/xfrm6_mode_beet.o
    172      32       0     204 net/ipv6/xfrm6_mode_ro.o
    172      32       0     204 net/ipv6/xfrm6_mode_transport.o
    599      32       0     631 net/ipv6/xfrm6_mode_tunnel.o

v2: pass inner_mode to xfrm_inner_mode_encap_remove to fix
    AF_UNSPEC selector breakage (bisected by Benedict Wong)

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