ipv6: fix rt6_update_expires

Commit 1716a961

 (ipv6: fix problem with expired dst cache) broke PMTU
discovery. rt6_update_expires() calls dst_set_expires(), which only updates
dst->expires if it has not been set previously (expires == 0) or if the new
expires is earlier than the current dst->expires.

rt6_update_expires() needs to zero rt->dst.expires, otherwise it will contain
ivalid data left over from rt->dst.from and will confuse dst_set_expires().

Signed-off-by: Jiri Bohac <jbohac@suse.cz>
Signed-off-by: David S. Miller <davem@davemloft.net>