xfrm: move the checking for old xfrm_policy hold_queue to beginning

if hold_queue of old xfrm_policy is NULL, return directly, then not need to
run other codes, especially take the spin lock

Signed-off-by: Li RongQing <roy.qing.li@gmail.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>