commit e819f152 upstream.

- Remove the unneeded declaration from pnode.h
- Mark umount_tree static as it has no callers outside of namespace.c
- Define an enumeration of umount_tree's flags.
- Pass umount_tree's flags in by name

This removes the magic numbers 0, 1 and 2 making the code a little
clearer and makes it possible for there to be lazy unmounts that don't
propagate.  Which is what __detach_mounts actually wants for example.
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
[ luis: backported to 3.16:
  - dropped change to __detach_mounts() ]
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>