Deal with revoke()'d descriptors. The underlying vnode is vgone()'d, which

Deal with revoke()'d descriptors.  The underlying vnode is vgone()'d, which
means that it is forceably closed.  This clears v_rdev even though file
descriptors may still have references to the vnode, which leads to panics
(mainly on tty devices).

Check for v_rdev == NULL in most specfs fileops calls and return an error
code instead of panicing.   This isn't the cleanest solution but it's fine
for now.

Reported-by: Sascha Wildner <saw@online.de>