network: fix handling of routing policy rule fwmask

The firewall mask should be applied even if the mark is 0, to allow defining a
value of e.g. 0/255.

Fixes #36973

test/test-network/conf/25-routing-policy-rule-test1.network

@@ -48,6 +48,24 @@ From=10.1.0.0/16
 Priority=104
 Table=12
 
+[RoutingPolicyRule]
+IncomingInterface=test1
+FirewallMark=0/1
+Priority=200
+Table=20
+
+[RoutingPolicyRule]
+IncomingInterface=test1
+FirewallMark=7/255
+Priority=201
+Table=21
+
+[RoutingPolicyRule]
+IncomingInterface=test1
+FirewallMark=9999
+Priority=202
+Table=22
+
 # The four routing policy rules below intentionally have the same config
 # excepts for their To= addresses. See issue #35874.
 [RoutingPolicyRule]

test/test-network/systemd-networkd-tests.py

@@ -3890,6 +3890,18 @@ class NetworkdNetworkTests(unittest.TestCase, Utilities):
         print(output)
         self.assertIn('104:	from 10.1.0.0/16 iif test1 lookup 12 nop', output)
 
+        output = check_output('ip rule list iif test1 priority 200')
+        print(output)
+        self.assertIn('200:	from all fwmark 0/0x1 iif test1 lookup 20', output)
+
+        output = check_output('ip rule list iif test1 priority 201')
+        print(output)
+        self.assertIn('201:	from all fwmark 0x7/0xff iif test1 lookup 21', output)
+
+        output = check_output('ip rule list iif test1 priority 202')
+        print(output)
+        self.assertIn('202:	from all fwmark 0x270f iif test1 lookup 22', output)
+
         output = check_output('ip rule list to 192.0.2.0/26')
         print(output)
         self.assertIn('to 192.0.2.0/26 lookup 1001', output)