lsns: interpolate missing namespaces for converting forests to a tree

lsns: interpolate missing namespaces for converting forests to a tree

The tree of *parent* and *owner* could be forests because *lsns*
cannot track a namespace having no process.

This change tries interpolating the missing namespaces by calling
ioctl(NS_GET_PARENT) and ioctl(NS_GET_USERNS) recursively.

The original output for -Tparent:

    # ./lsns -Tparent
    NS             TYPE   NPROCS   PID USER   COMMAND
    4026531837     user      404     1 root   /usr/lib/systemd/sy
    ├─4026532508   user        1 29376 yamato /usr/lib64/firefox/
    ...
    └─4026533513   user        1 24245 yamato /usr/lib64/firefox/
    ...
    4026533733     user        1 30839 yamato /opt/google/chrome-
    4026533734     user       15 10076 yamato /opt/google/chrome-

user namespaces 4026533733 and 4026533734 are orphans.
lsns could not find their parents.

With this change:

    # ./lsns-with-changes -Tparent
    NS               TYPE   NPROCS   PID USER   COMMAND
    4026531837       user      404     1 root   /usr/lib/systemd/
    ├─4026532508     user        1 29376 yamato /usr/lib64/firefo
    ...
    ├─4026532639     user        0
    │ ├─4026532637   user        0
    │ │ └─4026533733 user        1 30839 yamato /opt/google/chrom
    │ └─4026533734   user       14 10076 yamato /opt/google/chrom

Now user namespaces 4026533733 and 4026533734 are integrated to the
tree. lsns interpolates the missing namespace 4026532639 and
4026532637 for the integration.

The original output for -Towner:

    # ./lsns -Towner
    NS             TYPE   NPROCS   PID USER   COMMAND
    4026531837     user      405     1 root   /usr/lib/systemd/s
    ├─4026531835   cgroup    431     1 root   /usr/lib/systemd/s
    ...
    4026532638     pid         1 30839 yamato /opt/google/chrome
    4026532640     pid         2 30837 yamato /opt/google/chrome
    ...

pid namespaces 4026532638 and 4026532640 are orphans.
lsns could not find their owners.

With this change:

    # ./lsns-with-changes -Towner
    NS               TYPE   NPROCS   PID USER   COMMAND
    4026531837       user      403     1 root   /usr/lib/systemd
    ├─4026531835     cgroup    429     1 root   /usr/lib/systemd
    ...
    ├─4026532639     user        0
    ...
    │ ├─4026532637   user        0
    │ │ ├─4026532638 pid         1 30839 yamato /opt/google/chro
    │ │ ├─4026533638 net         1 30839 yamato /opt/google/chro
    │ │ └─4026533733 user        1 30839 yamato /opt/google/chro
    │ ├─4026532640   pid         2 30837 yamato /opt/google/chro

Now pid namespaces 4026532638 and 4026532640 are integrated to the
tree. lsns interpolates the missing namespace 4026532639 and
4026532637 for the integration.

Signed-off-by: Masatake YAMATO <yamato@redhat.com>