net: Set ping saddr after we successfully get the ping port

We can defer set ping saddr until we successfully get the ping port. So we
can avoid clear saddr when failed. Since ping_clear_saddr() is not used
anymore now, remove it.

Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>