ipv6: Rename the dst_cache helper functions in ip6_tunnel

It is a prep work to fix the dst_entry refcnt bugs in
ip6_tunnel.

This patch rename:
1. ip6_tnl_dst_check() to ip6_tnl_dst_get() to better
   reflect that it will take a dst refcnt in the next patch.
2. ip6_tnl_dst_store() to ip6_tnl_dst_set() to have a more
   conventional name matching with ip6_tnl_dst_get().

Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: David S. Miller <davem@davemloft.net>