verification/rvgen: Use .old instead of .bak for kunit backup files

The rvgen kunit command generates .bak backup files and these can be
checked in for selftests (make check). Clean targets like make disclean
remove such files, leaving the tree dirty.

Switch to .old to preserve a clean tree after make disclean.

Reported-by: Kuan-Wei Chiu <visitorckw@gmail.com>
Closes: https://lore.kernel.org/lkml/aosuwKH5GOEo0xTN@google.com
Fixes: 7b6246294e ("verification/rvgen: Add selftests for rvgen kunit")
Reviewed-by: Nam Cao <namcao@linutronix.de>
Tested-by: Kuan-Wei Chiu <visitorckw@gmail.com>
Link: https://lore.kernel.org/r/20260824081519.81103-2-gmonaco@redhat.com
Signed-off-by: Gabriele Monaco <gmonaco@redhat.com>

tools/verification/rvgen/rvgen/kunit.py

@@ -173,7 +173,7 @@ EXPORT_SYMBOL_IF_KUNIT({struct_name});
             for path in (header_file_path, kunit_c_file_path):
                 if path.exists():
                     try:
-                        path.rename(path.with_suffix(path.suffix + ".bak"))
+                        path.rename(path.with_suffix(path.suffix + ".old"))
                     except OSError as e:
                         raise KUnitError(f"Error backing up file {path}: {e}") from e