xfrm: remove input indirection from xfrm_mode

No need for any indirection or abstraction here, both functions
are pretty much the same and quite small, they also have no external
dependencies.

xfrm_prepare_input can then be made static.

With allmodconfig build, size increase of vmlinux is 25 byte:

Before:
   text   data     bss     dec      filename
15730207  6936924 4046908 26714039  vmlinux

After:
15730208  6936948 4046908 26714064 vmlinux

v2: Fix INET_XFRM_MODE_TRANSPORT name in is-enabled test (Sabrina Dubroca)
    change copied comment to refer to transport and network header,
    not skb->{h,nh}, which don't exist anymore. (Sabrina)
    make xfrm_prepare_input static (Eyal Birger)

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