Stable backport request for fixing the issue of not being able to create a new pid_ns

[Wen Yang]

After the process exits, the following three dentries still refer to the pid:
/proc/<pid>
/proc/<pid>/ns
/proc/<pid>/ns/pid

https://bugzilla.kernel.org/show_bug.cgi?id=208613

According to the commit f333c700c610 ("pidns: Add a limit on the number of
pid namespaces"), if the pid cannot be released, it may result in the
inability to create a new pid_ns.

Please backport the following patches to the kernel stable trees (from 4.9.y
to 5.6.y):
7bc3e6e55acf ("proc: Use a list of inodes to flush from proc")
26dbc60f385f ("proc: Generalize proc_sys_prune_dcache into proc_prune_siblings_dcache")
f90f3cafe8d5 ("proc: Use d_invalidate in proc_prune_siblings_dcache")

Signed-off-by: Wen Yang <wenyang@xxxxxxxxxxxxxxxxx>
Cc: Eric W. Biederman <ebiederm@xxxxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx