For an established TCP connection, the send queue is restored in two
steps: in step (1), we retransmit the data that was sent before but not
yet acknowledged, and in step (2), we transmit the data that was never
sent outside before.  The TCP_REPAIR option is disabled before step (2)
and re-enabled after step (2) (without this patch).

If the amount of data to be sent in step (2) is large, the TCP_REPAIR
flag on the socket can remain off for some time (O(milliseconds)).  If a
listen() is called on another socket bound to the same port during this
time window, it fails. This is because -- turning TCP_REPAIR off clears
the SO_REUSEADDR flag on the socket.

This patch adds a mutex (reuseaddr_lock) per port number, so that a
listen() on a port number does not happen while SO_REUSEADDR for another
socket on the same port is off.

Thanks to Amey Deshpande <ameyd@google.com> for debugging.
Signed-off-by: Saied Kazemi <saied@google.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>