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it's enough to set ->mnt_ns of internal vfsmounts to something
distinct from all struct mnt_namespace out there; then we can
just use the check for ->mnt_ns != NULL in the fast path of
mntput_no_expire()
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
(cherry picked from commit f7a99c5b7c8bd3d3f533c8b38274e33f3da9096e)