mptcp: balia: some bug-fixes

I found some possible bugs from the Balia code:

- The current subflow should be able to send (i.e., 'rate' should be
  non-zero) to calculate a new 'ai'.
  - In some extreme cases, e.g., there can be some subflows whose rate
    is almost 2^30. This may lead to u64 overflow by (sum_rate^2 * 10).
    So I reduce the
    upper-cap of the max_rate from 2^30 to 2^25.

Fixes: 075df3a63833 (mptcp: Add balia congestion control)
Signed-off-by: Jaehyun Hwang <jh.hwang@alcatel-lucent.com>
Signed-off-by: Christoph Paasch <christoph.paasch@gmail.com>