drm/amd/ras: chunk UNIRAS CPER debugfs reads

drm/amd/ras: chunk UNIRAS CPER debugfs reads

Legacy CPER ring readers can issue one debugfs read with a buffer larger
than the UNIRAS RAS command payload limit. Passing that full size to
GET_CPER_RECORD makes the command reject the request, so userspace may
only see the ring prefix and treat the CPER stream as empty.

Commit 3c88fb7aa57d ("drm/amd/ras: bound CPER record fetch buffer
size") intentionally bounds CPER record fetch allocation by the command
buffer size. Keep the debugfs ABI as a single contiguous ring read by
splitting the internal GET_CPER_RECORD requests into
RAS_CMD_MAX_CPER_BUF_SZ chunks.

Accumulate the copied payload and update the legacy header write pointers
from the total bytes returned to userspace.

Signed-off-by: Xiang Liu <xiang.liu@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>