sctp: do not loose window information if in rwnd_over

It's possible that we receive a packet that is larger than current
window. If it's the first packet in this way, it will cause it to
increase rwnd_over. Then, if we receive another data chunk (specially as
SCTP allows you to have one data chunk in flight even during 0 window),
rwnd_over will be overwritten instead of added to.

In the long run, this could cause the window to grow bigger than its
initial size, as rwnd_over would be charged only for the last received
data chunk while the code will try open the window for all packets that
were received and had its value in rwnd_over overwritten. This, then,
can lead to the worsening of payload/buffer ratio and cause rwnd_press
to kick in more often.

The fix is to sum it too, same as is done for rwnd_press, so that if we
receive 3 chunks after closing the window, we still have to release that
same amount before re-opening it.

Log snippet from sctp_test exhibiting the issue:
[  146.209232] sctp: sctp_assoc_rwnd_decrease: asoc:ffff88013928e000
rwnd decreased by 1 to (0, 1, 114221)
[  146.209232] sctp: sctp_assoc_rwnd_decrease:
association:ffff88013928e000 has asoc->rwnd:0, asoc->rwnd_over:1!
[  146.209232] sctp: sctp_assoc_rwnd_decrease: asoc:ffff88013928e000
rwnd decreased by 1 to (0, 1, 114221)
[  146.209232] sctp: sctp_assoc_rwnd_decrease:
association:ffff88013928e000 has asoc->rwnd:0, asoc->rwnd_over:1!
[  146.209232] sctp: sctp_assoc_rwnd_decrease: asoc:ffff88013928e000
rwnd decreased by 1 to (0, 1, 114221)
[  146.209232] sctp: sctp_assoc_rwnd_decrease:
association:ffff88013928e000 has asoc->rwnd:0, asoc->rwnd_over:1!
[  146.209232] sctp: sctp_assoc_rwnd_decrease: asoc:ffff88013928e000
rwnd decreased by 1 to (0, 1, 114221)

Signed-off-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>