accel/ethosu: reject DMA commands with uninitialized length

In the Linux kernel, the following vulnerability has been resolved:

accel/ethosu: reject DMA commands with uninitialized length

cmd_state_init() initializes the command state with memset(0xff),

leaving dma->len at U64_MAX to signal missing setup. The only setter

is NPU_SET_DMA0_LEN; if userspace omits this command and issues

NPU_OP_DMA_START, dma->len remains U64_MAX.

In dma_length(), a positive stride added to U64_MAX wraps to a small

value. With size0 == 1, check_mul_overflow() does not trigger and

dma_length() returns 0 instead of U64_MAX. The caller's U64_MAX check

then passes, region_size[] stays 0, and the bounds check in

ethosu_job.c is bypassed, allowing hardware to execute DMA with stale

physical addresses.

Fix by checking for U64_MAX at the start of dma_length() before any

arithmetic, consistent with the sentinel value used throughout the

driver to detect uninitialized fields.