xfrm: split xfrm_state_migrate into create and install functions

xfrm: split xfrm_state_migrate into create and install functions

To prepare for subsequent patches, split
xfrm_state_migrate() into two functions:
- xfrm_state_migrate_create(): creates the migrated state
- xfrm_state_migrate_install(): installs it into the state table

splitting will help to avoid SN/IV reuse when migrating AEAD SA.

And add const whenever possible.
No functional change.

Reviewed-by: Sabrina Dubroca <sd@queasysnail.net>
Signed-off-by: Antony Antony <antony.antony@secunet.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>