kernel - Remove improper EBUSY error on mount

* Remove the old VMOUNT vnode flag.  This flag is no longer applicable
  to anything we do.

* Simultanious mounts onto the same target at different points in the
  topology could cause races against the VMOUNT flag due to the vnode
  being shared, resulting in an EBUSY error for a perfectly good mount
  request.

  For example, if you mount_null A to X, A to Y, and A to Z, then
  you simultaniously try to mount devfs onto X/dev, Y/dev, and Z/dev,
  some of those dev mount attempts could fail.  This was due to the
  VMOUNT flag set on the vnode.  X/dev, Y/dev, and Z/dev share the
  same physical vnode.

* Fixes poudriere weirdness when many jails are in use.
kernel - Remove improper EBUSY error on mount (2)

* Fix lock order reversal in checkdirs() exposed by the removal
  of the VMOUNT flag.