Firstly, the bitmap we use for replay protection was ending up with zero

length, so a _single_ pair of packets getting switched around would
cause one of them to be 'dropped'.

Secondly, it wasn't even _dropping_ the offending packets, in the
non-blocking case. It was just returning garbage instead.
PR: #1752
Submitted by: David Woodhouse <dwmw2@infradead.org>