[stable 4.4, 4.9, 4.14, 4.19 LTS] Missing fix "memcg: fix a crash in wb_workfn when a device disappears"

Hi,

While reconciling the lttng-modules writeback instrumentation with its counterpart within the upstream Linux kernel, I notice that the following commit introduced in 5.6 is present in stable branches 5.4 and 5.5, but is missing from LTS stable branches for 4.4, 4.9, 4.14, 4.19:

commit 68f23b89067fdf187763e75a56087550624fdbee ("memcg: fix a crash in wb_workfn when a device disappears")

Considering that this fix was CC'd to the stable mailing list, is there any reason why it has not been integrated into those LTS branches ?

Thanks,

Mathieu