net: remove unnecessary input parameter 'how' in ifdown function

When the ifdown function in the dst_ops structure is referenced, the input
parameter 'how' is always true. In the current implementation of the
ifdown interface, ip6_dst_ifdown does not use the input parameter 'how',
xfrm6_dst_ifdown and xfrm4_dst_ifdown functions use the input parameter
'unregister'. But false judgment on 'unregister' in xfrm6_dst_ifdown and
xfrm4_dst_ifdown is false, so remove the input parameter 'how' in ifdown
function.

Signed-off-by: Zhengchao Shao <shaozhengchao@huawei.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://lore.kernel.org/r/20230821084104.3812233-1-shaozhengchao@huawei.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>