dm table: propagate BDI_CAP_STABLE_WRITES to fix sporadic checksum errors

commit eb40c0ac

 upstream.

Some devices don't use blk_integrity but still want stable pages
because they do their own checksumming.  Examples include rbd and iSCSI
when data digests are negotiated.  Stacking DM (and thus LVM) on top of
these devices results in sporadic checksum errors.

Set BDI_CAP_STABLE_WRITES if any underlying device has it set.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
[bwh: Backported to 3.16: request_queue::backing_dev_info is a struct
 not a pointer]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>