device: receive: drain decryption queue before exiting RoutineDecryption

device: receive: drain decryption queue before exiting RoutineDecryption

It's possible for RoutineSequentialReceiver to try to lock an elem after
RoutineDecryption has exited. Before this meant we didn't then unlock
the elem, so the whole program deadlocked.

As well, it looks like the flush code (which is now potentially
unnecessary?) wasn't properly dropping the buffers for the
not-already-dropped case.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>