xen: request-processing loop is unbounded in blkback

The doblockioop function in 1 drivers/xen/blkback/blkback.c and 2 drivers/xen/blktap/blktap.c in Xen before 3.4.0 for the Linux kernel 2.6.18, and possibly other versions, allows guest OS users to cause a denial of service infinite loop and CPU consumption via a large production request index to...