fix exception when using filesystem RA with DRBD proxy

(when proxy is on other node than DRBD)

exception:
java.lang.ClassCastException: lcmc.gui.resources.ProxyHostInfo cannot be
cast to lcmc.gui.resources.DrbdResourceInfo
lcmc.gui.resources.ProxyHostInfo cannot be cast to
lcmc.gui.resources.DrbdResourceInfo
lcmc.gui.resources.ServiceInfo.getCommonBlockDevInfos(ServiceInfo.java:1474)
lcmc.gui.resources.FilesystemInfo.createWidget(FilesystemInfo.java:242)
lcmc.gui.resources.EditableInfo.addParams(EditableInfo.java:339)
lcmc.gui.resources.EditableInfo.addParams(EditableInfo.java:312)
lcmc.gui.resources.ServiceInfo.getInfoPanel(ServiceInfo.java:2844)
lcmc.gui.Browser.getInfoPanel(Browser.java:231)