If I or kernel on reboot tries to umount a filesytem but its
underlying device is already gone (eg, due to vdevgone), we get panic
due to null pointer dereference.

The unmount vfs call tries to clear SI_MOUNTEDON bit of v_specflags,
but v_specinfo is already deallocated and NULL is assgined.

So, what is the right way to avoid this?  If I check vnode type and
make sure it is not VBAD before touching v_specflags prevent me from
crash, but I wonder if it is right way or not...

enami.