linuxkpi: Include `linux/sched/mm.h` from `linux/sched.h`

[dumbbell]

Summary

At least one file in the DRM drivers benefits from some namespace pollution to use fs_reclaim_acquire()/fs_reclaim_release(). They are defined in linux/sched/mm.h and this header must be included indirectly into the DRM drivers' source file.

I couldn't find how it was included. Therefore this commit includes linux/sched/mm.h from linux/sched.h. This is not the case in Linux but fixes the issue with the DRM drivers.

This is part of the update of the DRM drivers to Linux 5.13.

[dumbbell]

Re-upload patch with context.

[manu]

Maybe it will be easier/less problematic to include sched/mm.h in the problematic file in drm-kmod.

[dumbbell]

In D37912#861928, @manu wrote:

Maybe it will be easier/less problematic to include sched/mm.h in the problematic file in drm-kmod.

In fact, I did the same in D38154 (merged). I admit I would prefer to keep the problem where it is, in linuxkpi :-)