BACKPORT: xfrm: Provide API to register translator module

Add a skeleton for xfrm_compat module and provide API to register it in
xfrm_state.ko. struct xfrm_translator will have function pointers to
translate messages received from 32-bit userspace or to be sent to it
from 64-bit kernel.
module_get()/module_put() are used instead of rcu_read_lock() as the
module will vmalloc() memory for translation.
The new API is registered with xfrm_state module, not with xfrm_user as
the former needs translator for user_policy set by setsockopt() and
xfrm_user already uses functions from xfrm_state.

Signed-off-by: Dmitry Safonov <dima@arista.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>
(cherry picked from commit c9e7c76d70fa50582ca96759829c93d0dd024662)
[adelva: Edited around some context changes]
Bug: 163141236
Signed-off-by: Alistair Delva <adelva@google.com>
Change-Id: Ic825c6a0367fa192cc3f7af6b7d2682ef8f9d58b