Revert "xsk: simplify AF_XDP socket teardown"

Revert "xsk: simplify AF_XDP socket teardown"

This reverts commit a6d166d5d6ad3ced1bc814be98b04a5ecda2d41d.

It turns out that the sock destructor xsk_destruct was needed after
all. The cleanup simplification broke the skb transmit cleanup path,
due to that the umem was prematurely destroyed.

The umem cannot be destroyed until all outstanding skbs are freed,
which means that we cannot remove the umem until the sk_destruct has
been called.

Signed-off-by: Björn Töpel <bjorn.topel@intel.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>