net: tcp: add mib counters to track zero window transitions

Three counters are added:
- one to track when we went from non-zero to zero window
- one to track the reverse
- one counter incremented when we want to announce zero window,
  but can't because we would shrink current window.

Suggested-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --cc include/uapi/linux/snmp.h
Simple merge

diff --cc net/ipv4/proc.c
Simple merge

diff --cc net/ipv4/tcp_output.c
Simple merge