ipv6: introduce ip6_rt_put()

ipv6: introduce ip6_rt_put()

As suggested by Eric, we could introduce a helper function
for ipv6 too, to avoid checking if rt is NULL before
dst_release().

Cc: Eric Dumazet <[email protected]>
Cc: David S. Miller <[email protected]>
Signed-off-by: Cong Wang <[email protected]>
Signed-off-by: David S. Miller <[email protected]>