net: tcp: Do buf ref when resending a segment

We need to check whether buf_sent was true when resending the TCP
segment, and do a buf ref if needed. If this is not done, the buf
will be unref after send, which will cause unpredictable results.

Change-Id: Ibd4490305de88ac6ffd04ec42bba196e57da5c10
Signed-off-by: june li <junelizh@foxmail.com>