connstats currently increments the packet counter whenever it is called
to store a length of data, however when udp batch sending was introduced
we pass the length for a series of packages, and it is only incremented
ones, making it count wrongly if we are on a platform supporting udp
batches.
Updates tailscale/corp#22075
Signed-off-by: Kristoffer Dalby <kristoffer@tailscale.com>