drm/panthor: Prevent potential UAF in group creation

In the Linux kernel, the following vulnerability has been resolved: drm/panthor: Prevent potential UAF in group creation This commit prevents the possibility of a use after free issue in the GROUP_CREATE ioctl function, which arose as pointer to the group is accessed in that ioctl function after storing it in the Xarray. A malicious userspace can second guess the handle of a group and try to call GROUP_DESTROY ioctl from another thread around the same time as GROUP_CREATE ioctl. To prevent the use after free exploit, this commit uses a mark on an entry of group pool Xarray which is added just before returning from the GROUP_CREATE ioctl function. The mark is checked for all ioctls that specify the group handle and so userspace won't be abe to delete a group that isn't marked yet. v2: Add R-bs and fixes tags