Bugfix for plab nodes. This problem was tripped by Kevin. What was

happening was that when Kevin swapmod'd to get rid of failed nodes,
he just took the bad ones out.  This forced a change in the
vname<->vnode mapping, and the failed node got put in a state
(RES_INIT_CLEAN) that vnode_setup couldn't handle for plab nodes.
Basically, the problem is that vnode_setup was assuming that the
RES_INIT_CLEAN meant that the plab vnode needed to be allocated --
but it was already allocated in the previous swap.