vnet: Fix panic when shutting down jails and deleting interfaces simultaneously

[kp]

Summary

When we shut down a vnet jail its interfaces get moved back into their
original vnet. If we simultaneously delete that interface there's a
possible race where the interface is being moves while it's being
destroyed. That can result in if_vmove() attaching the interface into
the original vnet, and immediately freed. We then have a freed ifp in
V_ifnet, which will panic the next time we end up accessing it.

Don't re-attach dying interfaces.

This is frequently triggered by the automated pf tests.

[olivier]

Once this patch applied to a current, my build kernel failed with a

--- if.o ---
/usr/src/sys/net/if.c:1313:3: error: non-void function 'if_vmove' should return a value [-Wreturn-type]
                return;

[kp]

This will likely (I've not tested) fix the compile error, but fundamentally this patch is not the right solution to the underlying problem.

diff --git a/sys/net/if.c b/sys/net/if.c
index 2a80a0bc46e..ffa1129c06a 100644
--- a/sys/net/if.c
+++ b/sys/net/if.c
@@ -1308,6 +1308,10 @@ if_vmove(struct ifnet *ifp, struct vnet *new_vnet)
        if (ifp->if_reassign != NULL)
                ifp->if_reassign(ifp, new_vnet, NULL);

+       /* Don't re-attach DYING interfaces. */
+       if (ifp->if_flags & IFF_DYING)
+               return (0);
+
        /*
         * Switch to the context of the target vnet.
         */