[XFRM] net/xfrm/xfrm_state.c: use LIST_HEAD instead of LIST_HEAD_INIT

single list_head variable initialized with LIST_HEAD_INIT could almost
always can be replaced with LIST_HEAD declaration, this shrinks the code
and looks better.

Signed-off-by: Denis Cheng <crquan@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>