[mxfp] fix x_scale OOB (#8369)

```
pytest python/triton_kernels/tests/test_matmul.py::"test_op[True-True-True-True-False-None-16-300-400-800-ragged-mxfloat8_e4m3fn-mxfloat4_e2m1-8-4-1-True-None-False-False-False]"
```

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Author: jongsoo-openai

python/triton_kernels/triton_kernels/matmul_ogs_details/_p_matmul_ogs.py

@@ -311,7 +311,8 @@ def _p_matmul_ogs(
                         mask_k_scale = tl.full([MX_SCALE_BLOCK_K], True, dtype=tl.int1)
                     else:
                         mask_k_scale = off_k_mx + tl.arange(0, MX_SCALE_BLOCK_K) < tl.cdiv(K, MX_PACK_DIVISOR)
-                    x_scales = tl.load(XMxScalePtrs, mask=mask_k_scale[None, :], other=0.0)
+                    mask_m = off_m + tl.arange(0, BLOCK_M) < eM
+                    x_scales = tl.load(XMxScalePtrs, mask=mask_k_scale[None, :] & mask_m[:, None], other=0.0)
                 elif x_format == "fp16" or x_format == "bf16":
                     x_scales: tl.constexpr = None
                 else: