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 This flag is no longer applicable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.