xfrm: add state synchronization after migration

Add xfrm_migrate_sync() to copy curlft and replay state from the old SA
to the new one before installation. The function allocates no memory, so
it can be called under a spinlock. In preparation for a subsequent patch
in this series.

A subsequent patch calls this under x->lock, atomically capturing the
latest lifetime counters and replay state from the original SA and
deleting it in the same critical section to prevent SN/IV reuse
for XFRM_MSG_MIGRATE_STATE method.

No functional change.

Signed-off-by: Antony Antony <antony.antony@secunet.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>