xfrm: optimise the use of walk list header in xfrm_policy/state_walk

The walk from input is the list header, and marked as dead, and will
be skipped in loop.

list_first_entry() can be used to return the true usable value from
walk if walk is not empty

Signed-off-by: Li RongQing <roy.qing.li@gmail.com>
Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>