tcmu: Make dev_size configurable via userspace

Allow tcmu backstores to be able to set the device size
after it has been configured via set attribute.

Part of support in userspace to support certain backstores
changing device size.

Signed-off-by: Bryant G. Ly <bryantly@linux.vnet.ibm.com>
Reviewed-By: Mike Christie <mchristi@redhat.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>