Revert "udp: do rmem bulk free even if the rx sk queue is empty"

This reverts commit 0d4a6608.

Williem reported that after commit 0d4a6608 ("udp: do rmem bulk
free even if the rx sk queue is empty") the memory allocated by
an almost idle system with many UDP sockets can grow a lot.

For stable kernel keep the solution as simple as possible and revert
the offending commit.

Reported-by: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
Diagnosed-by: Eric Dumazet <eric.dumazet@gmail.com>
Fixes: 0d4a6608 ("udp: do rmem bulk free even if the rx sk queue is empty")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Acked-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>