Add an option to turn off discard and write zeroes offload support to
avoid deprovisioning a fully provisioned image.  When enabled, discard
requests will fail with -EOPNOTSUPP, write zeroes requests will fall
back to manually zeroing.
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Tested-by: Hitoshi Kamei <hitoshi.kamei.xm@hitachi.com>