Fix interfaces_file for proper file contents (#37818)

The generated file was completely unusable by the system
therefore the fix which ensures that diffing the file
prior to changes and after only shows diffs

Furthermore the code did not work for Python 3.6
>       f.writelines(to_bytes(lines, errors='surrogate_or_strict'))
E       TypeError: a bytes-like object is required, not 'int'

The other modifications (lambda variable renaming) is to
comply with default flake8 rules

lib/ansible/modules/system/interfaces_file.py

@@ -290,11 +290,11 @@ def setInterfaceOption(module, lines, iface, option, raw_value, state):
             if option in ["pre-up", "up", "down", "post-up"] and value is not None and value != "None":
                 for target_option in filter(lambda i: i['value'] == value, target_options):
                     changed = True
-                    lines = list(filter(lambda l: l != target_option, lines))
+                    lines = list(filter(lambda ln: ln != target_option, lines))
             else:
                 changed = True
                 for target_option in target_options:
-                    lines = list(filter(lambda l: l != target_option, lines))
+                    lines = list(filter(lambda ln: ln != target_option, lines))
     else:
         module.fail_json(msg="Error: unsupported state %s, has to be either present or absent" % state)
 
@@ -322,7 +322,7 @@ def write_changes(module, lines, dest):
 
     tmpfd, tmpfile = tempfile.mkstemp()
     f = os.fdopen(tmpfd, 'wb')
-    f.writelines(to_bytes(lines, errors='surrogate_or_strict'))
+    f.write(to_bytes(''.join(lines), errors='surrogate_or_strict'))
     f.close()
     module.atomic_move(tmpfile, os.path.realpath(dest))