xfrm: switch migrate to xfrm_policy_lookup_bytype

XFRM_MIGRATE still uses the old lookup method:
first check the bydst hash table, then search the list of all the other
policies.

Switch MIGRATE to use the same lookup function as the packetpath.

This is done to remove the last remaining users of the pernet
xfrm.policy_inexact lists with the intent of removing this list.

After this patch, policies are still added to the list on insertion
and they are rehashed as-needed but no single API makes use of these
anymore.

This change is compile tested only.

Cc: Tobias Brunner <tobias@strongswan.org>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>