With the typical model where the display server opens the file descriptor
and then hands it over to the client(*), we were showing stale data in
debugfs.

Fix it by updating the drm_file->pid on ioctl access from a different
process.

The field is also made RCU protected to allow for lockless readers. Update
side is protected with dev->filelist_mutex.

Before:

$ cat /sys/kernel/debug/dri/0/clients
             command   pid dev master a   uid      magic
                Xorg  2344   0   y    y     0          0
                Xorg  2344   0   n    y     0          2
                Xorg  2344   0   n    y     0          3
                Xorg  2344   0   n    y     0          4

After:

$ cat /sys/kernel/debug/dri/0/clients
             command  tgid dev master a   uid      magic
                Xorg   830   0   y    y     0          0
       xfce4-session   880   0   n    y     0          1
               xfwm4   943   0   n    y     0          2
           neverball  1095   0   n    y     0          3

*)
More detailed and historically accurate description of various handover
implementation kindly provided by Emil Velikov:

"""
The traditional model, the server was the orchestrator managing the
primary device node. From the fd, to the master status and
authentication. But looking at the fd alone, this has varied across
the years.

IIRC in the DRI1 days, Xorg (libdrm really) would have a list of open
fd(s) and reuse those whenever needed, DRI2 the client was responsible
for open() themselves and with DRI3 the fd was passed to the client.

Around the inception of DRI3 and systemd-logind, the latter became
another possible orchestrator. Whereby Xorg and Wayland compositors
could ask it for the fd. For various reasons (hysterical and genuine
ones) Xorg has a fallback path going the open(), whereas Wayland
compositors are moving to solely relying on logind... some never had
fallback even.

Over the past few years, more projects have emerged which provide
functionality similar (be that on API level, Dbus, or otherwise) to
systemd-logind.
"""

v2:
 * Fixed typo in commit text and added a fine historical explanation
   from Emil.
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: "Christian König" <christian.koenig@amd.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
Acked-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Rob Clark <robdclark@gmail.com>
Tested-by: Rob Clark <robdclark@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230621094824.2348732-1-tvrtko.ursulin@linux.intel.comSigned-off-by: Christian König <christian.koenig@amd.com>