rust: dma: return zero for Coherent reads past EOF

Coherent<T>::write_to_slice() calculates a zero-byte copy when the file
offset is beyond the allocation, but still calls
UserSliceWriter::write_dma(). The latter rejects offsets beyond the
allocation even when the copy length is zero, so a debugfs read past EOF
returns -ERANGE.

Return before calling write_dma() when the offset is at or beyond the
allocation, matching simple_read_from_buffer() EOF semantics.

Fixes: 0168185139 ("rust: dma: implement BinaryWriter for Coherent<[u8]>")
Cc: stable@vger.kernel.org
Link: https://rust-for-linux.zulipchat.com/#narrow/channel/291566-Library/topic/.E2.9C.94.20Possible.20past-EOF.20bug.20in.20Coherent.3CT.3E.3A.3Awrite_to_slice/near/611677095
Signed-off-by: Younes Akhouayri <git@younes.io>
Reviewed-by: Alexandre Courbot <acourbot@nvidia.com>
Reviewed-by: Onur Özkan <work@onurozkan.dev>
Link: https://patch.msgid.link/20260730-fix-dma-coherent-eof-v2-1-8aff21054afa@younes.io
Signed-off-by: Danilo Krummrich <dakr@kernel.org>

rust/kernel/dma.rs

@@ -967,7 +967,11 @@ fn write_to_slice(
             return Ok(0);
         };
 
-        let count = self.size().saturating_sub(offset_val).min(writer.len());
+        if offset_val >= self.size() {
+            return Ok(0);
+        }
+
+        let count = (self.size() - offset_val).min(writer.len());
 
         writer.write_dma(self, offset_val, count)?;