iscsi: fix activation of second iSCSI storage on other cluster nodes

Assume a cluster that already has an iSCSI storage A configured. After
adding a new iSCSI storage B with a different target on node 1, B will
only become active on node 1, not on the other nodes. On other nodes,
pvestatd logs 'storage B is not online'. The storage does not become
available even after a reboot. A workaround is to manually perform
iSCSI discovery against B's targets on the other nodes once.

This happens because the connectivity check of the iSCSI plugin on
node B does not correctly handle the case that iscsiadm already knows
portals (i.e., A's portals) but not B's portals.

The connectivity check calls `iscsi_portals` to determine the portals
to ping, which calls `iscsiadm -m node` to query all known portals,
and extracts all portals to the storage's target. If the iscsiadm
command fails, `iscsi_portals` returns the portal given in the storage
config. This works as expected if the storage is the first iSCSI
storage, because then iscsiadm does not know any portals and thus
exits with code 21.

However, since there already is an iSCSI storage A, iscsiadm exits
cleanly but its output does not contain any portals for B's target.
Hence, `iscsi_portals` returns an empty array of portals, so the
connectivity check fails and node 2 never performs discovery for B.

To fix this, let `iscsi_portals` also return the portal from B's
storage config if iscsiadm exited cleanly but its output contained no
matching portal.

Signed-off-by: Friedrich Weber <f.weber@proxmox.com>

src/PVE/Storage/ISCSIPlugin.pm

@@ -86,12 +86,14 @@ sub iscsi_portals {
 	});
     };
 
-    if ($@) {
-	warn $@;
-	return [ $portal_in ];
-    }
+    my $err = $@;
+    warn $err if $err;
 
-    return $res;
+    if ($err || !scalar(@$res)) {
+	return [ $portal_in ];
+    } else {
+	return $res;
+    }
 }
 
 sub iscsi_discovery {