Backport request for bcb44433bba5 ("dm: disable DISCARD if the underlying storage no longer supports it")

Hi

In Debian bug https://bugs.debian.org/934331 ran into issues which match the upstream commit bcb44433bba5 ("dm: disable DISCARD if the underlying storage no longer supports it").

This commit was CC'ed to stable, but only got applied in v5.0.8 (and later on backported by Ben Hutchings to v3.16.72).

Mike, I have not checked how easily that would be for older stable versions, but can the backport be considered for versions down to 4.9? Apparently Ben did succeed with some changes needed. To 4.19 it should apply with a small conflict in drivers/md/dm-core.h AFAICS.

Regards, Salvatore