device: prevent spurious errors while closing a device

device: prevent spurious errors while closing a device

When closing a device, packets that are in flight
can make it to SendBuffer, which then returns an error.
Those errors add noise but no light;
they do not reflect an actual problem.

Adding the synchronization required to prevent
this from occurring is currently expensive and error-prone.
Instead, quietly drop such packets instead of
returning an error.

Signed-off-by: Josh Bleecher Snyder <josh@tailscale.com>