Fix the interface name length validation for subinterface (#3799)

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#### What I did
Subinterface name length validation validates only the interface alias and not the entire subinterface name. This will result in wrong validation.

#### How I did it
Fixed the validation to use subinterface name and not just interface alias.

#### How to verify it
Added test case to verify.

#### Previous command output (if the output of a command-line utility has changed)

#### New command output (if the output of a command-line utility has changed)

Author: mssonicbld

config/main.py

@@ -8704,7 +8704,7 @@ def add_subinterface(ctx, subinterface_name, vid):
 
         if interface_alias is None:
             ctx.fail("{} invalid subinterface".format(interface_alias))
-        if not isInterfaceNameValid(interface_alias):
+        if not isInterfaceNameValid(subinterface_name):
             ctx.fail("Subinterface name length should not exceed {} characters".format(IFACE_NAME_MAX_LEN))
 
         if interface_alias.startswith("Po") is True:

tests/subintf_test.py

@@ -87,6 +87,13 @@ def test_add_existing_subintf_again(self):
         assert result.exit_code != 0
         assert ('Eth0.1002') not in db.cfgdb.get_table('VLAN_SUB_INTERFACE')
 
+        # Check if interface name length doesn't exceed 15 characters
+        result = runner.invoke(config.config.commands["subinterface"].commands["add"], ["Ethernet0.000002", "2"],
+                               obj=obj)
+        print(result.exit_code, result.output)
+        assert result.exit_code != 0
+        assert "Error: Subinterface name length should not exceed 15 characters" in result.output
+        assert ('Ethernet0.000002') not in db.cfgdb.get_table('VLAN_SUB_INTERFACE')
 
     def test_delete_non_existing_subintf(self):
         runner = CliRunner()