Looks good.

I've added prints in the new code paths and verified that:

No need to check shutdown flag in IPv6 fragment cleanup code yet (@bz fill add this later)

Fix check for shutdown in IPv4 reassembly cleanup.

Yes, that's the simplest.

Ultimately, why does this stuff need to be done asynchronously in a different process? I would understand if bits outside of dev/usb/serial/* were submitting commands to this process as well, but that's simply not the case as far as I can tell -- it looks like it was mostly done like this to match the structure of usb_ethernet, which is not a solid reason.

@bz: The old checks don't respect if we are bringing up or destroying a VNET. For example this one in if_detach:

sys/ofed, sys/dev/mlx4, sys/dev/mlx5 and sys/dev/usb changes look OK.

Update patch.

Rebase patch.

Submitted upstream.