don't allow enabling a write back cache on devices that don't support it

[Christoph Hellwig <hch@xxxxxx>]

Hi Jens,

this series fixes a case where a user could enable a write back cache
on a device (driver) that doesn't support it all, potentially leading
to flush requests being sent to a driver that doesn't know whast to
do with them.

diffstat:
 block/blk-settings.c   |    7 +++++--
 block/blk-sysfs.c      |   21 ++++++++-------------
 include/linux/blkdev.h |    1 +
 3 files changed, 14 insertions(+), 15 deletions(-)