drm/imagination: Size page table preallocation by device address

In the Linux kernel, the following vulnerability has been resolved: drm/imagination: Size page table preallocation by device address Commit 0a8224058a58 ("drm/imagination: Fix page count for page table for map() interface") passed the device address to pvr_mmu_op_context_create(), but the preallocation is still sized from device_addr + sgt_offset with an exclusive end. The offset into the object's pages has no place in a device-virtual range, and the exclusive end allocates one table too many when the range ends on a table boundary. Count the tables from device_addr and the inclusive end of the range. The new calculation does not take page table indices into account. This fixes another issue with the previous implementation, where the difference between start and end indices could underflow when a mapping crossed a page table boundary. This would lead to the driver preallocating a huge number of MMU pages, exhausting system memory and triggering an out of memory panic. [Alessio: add paragraph to point out OOM bug being fixed, cc stable]