accel/ethosu: fix arithmetic issues in dma_length()

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.

Fixes: 5a5e9c0228e6 ("accel: Add Arm Ethos-U NPU driver")
Cc: stable@vger.kernel.org
Signed-off-by: Muhammad Bilal <meatuni001@gmail.com>
Link: https://patch.msgid.link/20260524103710.47397-1-meatuni001@gmail.com
Signed-off-by: Rob Herring (Arm) <robh@kernel.org>