bpo-34754: Fix test_flush_return_value on FreeBSD (GH-9451)

Apparently, FreeBSD doesn't raise OSError when offset is
not a multiple of mmap.PAGESIZE.

Lib/test/test_mmap.py

@@ -749,8 +749,9 @@ class MmapTests(unittest.TestCase):
         mm.write(b'python')
         result = mm.flush()
         self.assertIsNone(result)
-        if os.name != 'nt':
-            # 'offset' must be a multiple of mmap.PAGESIZE.
+        if sys.platform.startswith('linux'):
+            # 'offset' must be a multiple of mmap.PAGESIZE on Linux.
+            # See bpo-34754 for details.
             self.assertRaises(OSError, mm.flush, 1, len(b'python'))