sctp: drop a_rwnd to 0 when receive buffer overflows.
SCTP has a problem that when small chunks are used, it is possible to exhaust the receiver buffer without fully closing receive window. This happens due to all overhead that we have account for with small messages. To fix this, when receive buffer is exceeded, we'll drop the window to 0 and save the 'drop' portion. When application starts reading data and freeing up recevie buffer space, we'll wait until we've reached the 'drop' window and then add back this 'drop' one mtu at a time. This worked well in testing and under stress produced rather even recovery. Signed-off-by: Vlad Yasevich <email@example.com>