ice: fix accounting if a VLAN already exists

The ice_vsi_add_vlan() function is used to add a VLAN filter for the target
VSI. This function prepares a filter in the switch table for the given VSI.
If it succeeds, the vsi->num_vlan counter is incremented.

It is not considered an error to add a VLAN which already exists in the
switch table, so the function explicitly checks and ignores -EEXIST. The
vsi->num_vlan counter is still incremented.

This seems incorrect, as it means we can double-count in the case where the
same VLAN is added twice by the caller. The actual table will have one less
filter than the count.

The ice_vsi_del_vlan() function similarly checks and handles the -ENOENT
condition for when deleting a filter that doesn't exist. This flow only
decrements the vsi->num_vlan if it actually deleted a filter.

The vsi->num_vlan counter is used only in a few places, primarily related
to tracking the number of non-zero VLANs. If the vsi->num_vlans gets out of
sync, then ice_vsi_num_non_zero_vlans() will incorrectly report more VLANs
than are present, and ice_vsi_has_non_zero_vlans() could return true
potentially in cases where there are only VLAN 0 filters left.

Fix this by only incrementing the vsi->num_vlan in the case where we
actually added an entry, and not in the case where the entry already
existed.

Fixes: a1ffafb0 ("ice: Support configuring the device to Double VLAN Mode")
Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Pucha Himasekhar Reddy <himasekharx.reddy.pucha@intel.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://lore.kernel.org/r/20240523-net-2024-05-23-intel-net-fixes-v1-2-17a923e0bb5f@intel.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/ethernet/intel/ice/ice_vsi_vlan_lib.c

@@ -45,14 +45,15 @@ int ice_vsi_add_vlan(struct ice_vsi *vsi, struct ice_vlan *vlan)
 		return -EINVAL;
 
 	err = ice_fltr_add_vlan(vsi, vlan);
-	if (err && err != -EEXIST) {
+	if (!err)
+		vsi->num_vlan++;
+	else if (err == -EEXIST)
+		err = 0;
+	else
 		dev_err(ice_pf_to_dev(vsi->back), "Failure Adding VLAN %d on VSI %i, status %d\n",
 			vlan->vid, vsi->vsi_num, err);
-		return err;
-	}
 
-	vsi->num_vlan++;
-	return 0;
+	return err;
 }
 
 /**