lwtunnel: add options process for cmp_encap

lwtunnel: add options process for cmp_encap

When comparing two tun_info, dst_cache member should have been skipped,
as dst_cache is a per cpu pointer and they are always different values
even in two tun_info with the same keys.

So this patch is to skip dst_cache member and compare the key, mode and
options_len only. For the future opts setting support, also to compare
options.

Fixes: 5d7890ec2a30 ("lwtunnel: ip tunnel: fix multiple routes with different encap")
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>