The length of a packet read from the underlying TUN device may exceed
the length of a supplied buffer when MTU exceeds device.MaxMessageSize.
Reviewed-by: Brad Fitzpatrick <bradfitz@tailscale.com>
Signed-off-by: Jordan Whited <jordan@tailscale.com>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>