buffer cache - Control all access to the buf red-black trees with vp->v_token

buffer cache - Control all access to the buf red-black trees with vp->v_token

Access to the buffer cache's RB trees is now controlled via vp->v_token
instead of a critical section.  We still hold the BGL but this is not
quite as simple as it seems because an interrupt calling biodone() on a
B_INVAL buffer may now potentially block, where as it would not have before.
The buffer is locked.