mntns: simplify a case, when a root should be changed
mntns: simplify a case, when a root should be changed

We don't need to clean_mnt_ns(), if we are goning to do pivot_root().

"""
pivot_root moves the root file system of the current process to the
directory put_old and makes new_root the new root file system.
"""

So I suggest to do pivot_root() and then detach the old root, all
other mount points will be unmounted automatically.

This patch fixes a problem, when a new root is mounted above a non-root
mount point. It's a default configuration for OpenVZ.

Signed-off-by: Andrey Vagin <74c05afa918858348d6db8037d82fa8124870113@openvz.org>
Signed-off-by: Pavel Emelyanov <c9a32589e048e044184536f7ac71ef92fe82df3e@parallels.com>

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