ipv6: remove unnecessary dst_hold() in ip6_fragment()

In ipv6 tx path, rcu_read_lock() is taken so that dst won't get freed
during the execution of ip6_fragment(). Hence, no need to hold dst in
it.

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