net: tcp: When retransmitting, hold an extra, temporary reference

The link layer dereferences a buffer right after it is transmitted.

If this extra reference is not held, the second time a buffer is
retransmitted, the reference that TCP holds when keeping the buffer in
the `sent_list` will be taken, and retransmission won't happen reliably
anymore.

As soon as the TCP fragment is acknowledged by the peer, the
`sent_list` reference is taken, and the buffer is freed.

Change-Id: Ie50f9acf02c1dff74248a5dfbec3785a91ff90f7
Signed-off-by: Leandro Pereira <leandro.pereira@intel.com>