Revert "xsk: simplify AF_XDP socket teardown"

Revert "xsk: simplify AF_XDP socket teardown"

This reverts commit e2ce3674883ecba2605370404208c9d4a07ae1c3.

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>