ANDROID: rename struct tcm_sock.cwnd_usage_seq to fix ABI

In commit 49d42976 ("tcp: fix tcp_cwnd_validate() to not forget
is_cwnd_limited"), the field max_packets_seq in struct tcm_sock was
renamed to cwnd_usage_seq, which broke the ABI.  Keep the same logic in
the bugfix, but rename the field back to preserve the ABI.

Bug: 161946584
Fixes: 49d42976 ("tcp: fix tcp_cwnd_validate() to not forget is_cwnd_limited")
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>
Change-Id: I9ffb10be63a22c36d5864feea794dde390ef69a1