xfrm: add and use xfrm_state_afinfo_get_rcu

xfrm_init_tempstate is always called from within rcu read side section.
We can thus use a simpler function that doesn't call rcu_read_lock
again.

While at it, also make xfrm_init_tempstate return value void, the
return value was never tested.

A followup patch will replace remaining callers of xfrm_state_get_afinfo
with xfrm_state_afinfo_get_rcu variant and then remove the 'old'
get_afinfo interface.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>