vsock/test: add MSG_PEEK after partial recv test

vsock/test: add MSG_PEEK after partial recv test

Add a test that verifies MSG_PEEK works correctly after a partial
recv().

This is to test a bug that was present in the
`virtio_transport_stream_do_peek()` when computing the number of bytes to
copy: After a partial read, the peek function didn't take into
consideration the number of bytes that were already read. So peeking the
whole buffer would cause an out-of-bounds read, that resulted in a -EFAULT.

This test does exactly this: do a partial recv on a buffer, then try to
peek the whole buffer content. The test re-uses
`test_stream_msg_peek_client()` to also cover this scenario.

Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: Luigi Leonardi <leonardi@redhat.com>
Link: https://patch.msgid.link/20260415-fix_peek-v4-3-8207e872759e@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>