tcp: add LINUX_MIB_PAWS_OLD_ACK SNMP counter

tcp: add LINUX_MIB_PAWS_OLD_ACK SNMP counter

Prior patch in the series added TCP_RFC7323_PAWS_ACK drop reason.

This patch adds the corresponding SNMP counter, for folks
using nstat instead of tracing for TCP diagnostics.

nstat -az | grep PAWSOldAck

Suggested-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Neal Cardwell <ncardwell@google.com>
Reviewed-by: Jason Xing <kerneljasonxing@gmail.com>
Tested-by: Neal Cardwell <ncardwell@google.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://patch.msgid.link/20250113135558.3180360-4-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>