In the spirit of commit 91751e24 ("vsock: prevent null-ptr-deref in
vsock_*[has_data|has_space]"), armorize the "impossible" cases with a
warning.

Fixes: 634f1a71 ("vsock: support sockmap")
Signed-off-by: Michal Luczaj <mhal@rbox.co>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>