[mgs] Don't drop the ereport socket (#8583)

Turns out that dropping this cancels the receive handler task for the
ereport socket, which results in ereport collection being totally
busted. This wasn't caught until I actually tested the system end-to-end
on a racklette with a real SP emitting ereports, since the omicron
integration tests don't actually use a single shared socket, and
`faux-mgs` just puts it on the stack, so none of this was actually
exercised outside of a production-like environment. Cool.

Shoutout to @mkeeter for finding the bug!

Author: hawkw

gateway/src/management_switch.rs

@@ -161,6 +161,8 @@ pub struct ManagementSwitch {
     // When it's dropped, it cancels the background tokio task that loops on
     // that socket receiving incoming packets.
     _shared_socket: Option<SharedSocket<shared_socket::SingleSpMessage>>,
+    // As above, we must also not drop the shared ereport socket.
+    _shared_ereport_socket: Option<SharedSocket<Vec<u8>>>,
     location_map: Arc<OnceLock<Result<LocationMap, String>>>,
     discovery_task: JoinHandle<()>,
     log: Logger,
@@ -336,6 +338,7 @@ impl ManagementSwitch {
             local_ignition_controller_port,
             location_map,
             _shared_socket: shared_socket,
+            _shared_ereport_socket: ereport_socket,
             port_to_handle,
             port_to_ignition_target,
             discovery_task,