sctp: Move sctp_v4_dst_saddr out of loop

There's no need to execute sctp_v4_dst_saddr() for each
iteration, just move it out of loop.

Signed-off-by: Gui Jianfeng <guijianfeng@cn.fujitsu.com>
Signed-off-by: Vlad Yasevich <vladislav.yasevich@hp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>