Add OF_xref_from_node_strict() which returns -1 if there is no xref handle
for the node.  The default routine returns the untranslated handle, which
is sometimes useful, but sometimes you really need to know there's no
entry in the xref<->node<->device translation table.


git-svn-id: svn+ssh://svn.freebsd.org/base/head@271190 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f