Fix: Please help to cherry-pick patch "bdi: Do not use freezable workqueue" to stable tree

[Macpaul Lin]

Dear Greg,

Our customers have feedback some similar issues as below link on Android
kernel-4.14 and kernel-4.19.
  Link: https://marc.info/?l=linux-kernel&m=138695698516487

They've reported system become abnormal when OTG U-disk has been plugged
out after the system has been suspended.
After debugging, we've found the root cause is the same of the issue
(link) has been reported. We've also tested the patch "bdi: Do not use
freezable workqueue" is worked.
  Link: https://lkml.org/lkml/2019/10/4/176
  commit id in Linus tree: a2b90f11217790ec0964ba9c93a4abb369758c26

However, we've checked that patch seems hasn't been applied to stable
tree (We've checked 4.14 and 4.19). Would you please help to cherry-pick
this patch to stable trees (and to Android trees)?

Thanks!
Macpaul Lin