vsock/virtio: Linger on unsent data

vsock/virtio: Linger on unsent data

Currently vsock's lingering effectively boils down to waiting (or timing
out) until packets are consumed or dropped by the peer; be it by receiving
the data, closing or shutting down the connection.

To align with the semantics described in the SO_LINGER section of man
socket(7) and to mimic AF_INET's behaviour more closely, change the logic
of a lingering close(): instead of waiting for all data to be handled,
block until data is considered sent from the vsock's transport point of
view. That is until worker picks the packets for processing and decrements
virtio_vsock_sock::bytes_unsent down to 0.

Note that (some interpretation of) lingering was always limited to
transports that called virtio_transport_wait_close() on transport release.
This does not change, i.e. under Hyper-V and VMCI no lingering would be
observed.

The implementation does not adhere strictly to man page's interpretation of
SO_LINGER: shutdown() will not trigger the lingering. This follows AF_INET.

Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: Michal Luczaj <mhal@rbox.co>
Link: https://patch.msgid.link/20250522-vsock-linger-v6-1-2ad00b0e447e@rbox.co
Signed-off-by: Paolo Abeni <pabeni@redhat.com>