Revert "vsock/loopback: use only sk_buff_head.lock to protect the packet queue"

This reverts commit bb1c9a59 which is
commit b465518d upstream.

It needs to be reverted, as the function virtio_transport_free_pkt() is
in the stable Android ABI, and this function is removed in the 6.1.63
kernel as part of a series of fixes and optimizations of the virtio
networking connection.  To preserve the ABI (and thereby keeping the
slower interface), revert the commit for now.

If this needs to come back in the future, if performance for this
interface needs to be resolved, it can be done so in an ABI-safe way.

Bug: 161946584
Change-Id: I5b21a61d82919196c5a386c25eed05055cf77714
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>