sock_map: fix a NULL pointer dereference in sock_map_link_update_prog()

In the Linux kernel, the following vulnerability has been resolved:

sock_map: fix a NULL pointer dereference in sock_map_link_update_prog()

The following race condition could trigger a NULL pointer dereference:

sock_map_link_detach(): sock_map_link_update_prog():

mutex_lock(&sockmap_mutex);

...

sockmap_link->map = NULL;

mutex_unlock(&sockmap_mutex);

mutex_lock(&sockmap_mutex);

...

sock_map_prog_link_lookup(sockmap_link->map);

mutex_unlock(&sockmap_mutex);

<continue>

Fix it by adding a NULL pointer check. In this specific case, it makes

no sense to update a link which is being released.