This patch refactors pkg/util/mount to be more usable outside of
Kubernetes. This is done by refactoring mount.Interface to only contain
methods that are not K8s specific. Methods that are not relevant to
basic mount activities but still have OS-specific implementations are
now found in a mount.HostUtils interface.
Since pkg/util/mount is going to move out of k/k, this exported constant
that is Kubernetes specific needed to move somewhere else. Made sense to
move it to pkg/volume/util.
Update GetDeviceNameFromMount in the mount interface to now take a
pluginMountDir argument, which is volume plugin dir with the global
mount path appended to it already.
As part of moving pkg/util/mount out of tree, the NSEnter implementation
of mount.Interface needs to be relocated out of pkg/util/mount, as it is
K8s specific. This patch relocates that mounter implementation to
pkg/volume/util/nsenter.
Since the NSEnter mounter shares a lot of its logic with the Linux
mounter implementation, many of the previously private methods of the
Linux mounter are now made public to maintain that shared code.
Additionaly, it was observed that *all* mount.Interface implemenations
were using the same common method for IsNotMountPoint, so this patch
removes that method from the mount.Interface definition and just exports
the common implementation instead.
