device: remove QueueInboundElement.dropped

device: remove QueueInboundElement.dropped

Now that we block when enqueueing to the decryption queue,
there is only one case in which we "drop" a inbound element,
when decryption fails.

We can use a simple, obvious, sync-free sentinel for that, elem.packet == nil.
Also, we can return the message buffer to the pool slightly later,
which further simplifies the code.

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