vsock/test: Fix occasional failure in SIOCOUTQ tests

vsock/test: Fix occasional failure in SIOCOUTQ tests

These tests:
    "SOCK_STREAM ioctl(SIOCOUTQ) 0 unsent bytes"
    "SOCK_SEQPACKET ioctl(SIOCOUTQ) 0 unsent bytes"
output: "Unexpected 'SIOCOUTQ' value, expected 0, got 64 (CLIENT)".

They test that the SIOCOUTQ ioctl reports 0 unsent bytes after the data
have been received by the other side. However, sometimes there is a delay
in updating this "unsent bytes" counter, and the test fails even though
the counter properly goes to 0 several milliseconds later.

The delay occurs in the kernel because the used buffer notification
callback virtio_vsock_tx_done(), called upon receipt of the data by the
other side, doesn't update the counter itself. It delegates that to
a kernel thread (via vsock->tx_work). Sometimes that thread is delayed
more than the test expects.

Change the test to poll SIOCOUTQ until it returns 0 or a timeout occurs.

Signed-off-by: Konstantin Shkolnyy <kshk@linux.ibm.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Fixes: 18ee44ce97c1 ("test/vsock: add ioctl unsent bytes test")
Link: https://patch.msgid.link/20250507151456.2577061-1-kshk@linux.ibm.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>