accel/ethosu: fix arithmetic issues in dma_length()

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

accel/ethosu: fix arithmetic issues in dma_length()

dma_length() derives DMA region usage from command stream values and

updates region_size[]:

len = ((len + stride[0]) * size0 + stride[1]) * size1

region_size[region] = max(..., len + dma->offset)

Several arithmetic issues can corrupt the derived region size:

- signed stride values may underflow when added to len

- intermediate multiplications may overflow

- len + dma->offset may overflow during region_size updates

- dma_length() error returns were not validated by the caller

region_size[] is later used by ethosu_job.c to validate command stream

accesses against GEM buffer sizes. Arithmetic wraparound can therefore

under-report region usage and bypass the bounds validation.

Fix by validating signed additions, using overflow helpers for

multiplications and offset updates, and propagating dma_length()

failures to the caller.