xfrm6: remove BUG_ON from xfrm6_dst_ifdown

if loopback_idev is NULL pointer, and the following access of
loopback_idev will trigger panic, which is same as BUG_ON

Signed-off-by: Li RongQing <lirongqing@baidu.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>