kernel/vfs: enable accounting for ro mounts

kernel/vfs: enable accounting for ro mounts

* Some filesystems may be first mounted read-only and then upgraded
  to read-write status

* The VFS layer has no provision for signaling VFSes of status change,
  mount-point specific initialization code can only be run for the
  initial mount

* VFS accounting was thus never enabled on a ro->rw mount point transition

* Bite the bullet and run vq_init() unconditionally for both read-only
  and read-write mounts, the cost is only a few variables set to NULL
  or zero anyway.