2/3 vfs: fix device count leak on vrele racing with vgone

[mjg]

Summary

The race is:

CPU1                                        CPU2
                                    devfs_reclaim_vchr
make v_usecount 0
                                      VI_LOCK
                                      sees v_usecount == 0, no updates
                                      vp->v_rdev = NULL;
                                      ...
                                      VI_UNLOCK
VI_LOCK
v_decr_devcount
  sees v_rdev == NULL, no updates

In this scenario si_devcount decrement is not performed.

Note this can only happen if the vnode lock is not held.

Also see the 3/3 review.

[mjg]

tested by pho