tcp: initialize rcv_mss to TCP_MIN_MSS instead of 0

tcp: initialize rcv_mss to TCP_MIN_MSS instead of 0

When tcp_disconnect() is called, inet_csk_delack_init() sets
icsk->icsk_ack.rcv_mss to 0.
This could potentially cause tcp_recvmsg() => tcp_cleanup_rbuf() =>
__tcp_select_window() call path to have division by 0 issue.
So this patch initializes rcv_mss to TCP_MIN_MSS instead of 0.

Reported-by: Andrey Konovalov  <[email protected]>
Signed-off-by: Wei Wang <[email protected]>
Signed-off-by: Eric Dumazet <[email protected]>
Signed-off-by: Neal Cardwell <[email protected]>
Signed-off-by: Yuchung Cheng <[email protected]>
Signed-off-by: David S. Miller <[email protected]>