Physical-memory access via stale SRAM size

Memory-mapping failure handling in the Linux kernel 6.19 Ethos-U driver allows local users to direct NPU jobs to physical address zero. ethosu_sram_init records the SRAM pool size before allocation and, on allocation failure, leaves that size nonzero while the physical pointer remains zero. Reachability requires an Arm Ethos-U device, an SRAM mapping failure, and permission to submit accelerator jobs; resulting jobs access an unintended physical region.