rxrpc: Fix local destruction being repeated

If the local processor work item for the rxrpc local endpoint gets requeued
by an event (such as an incoming packet) between it getting scheduled for
destruction and the UDP socket being closed, the rxrpc_local_destroyer()
function can get run twice.  The second time it can hang because it can end
up waiting for cleanup events that will never happen.

Signed-off-by: David Howells <dhowells@redhat.com>