kernel: drm/shmem-helper: Fix BUG_ON() on mmap(PROT_WRITE, MAP_PRIVATE)

In the Linux kernel, the following vulnerability has been resolved: drm/shmem-helper: Fix BUGON on mmapPROTWRITE, MAPPRIVATE Lack of check for copy-on-write COW mapping in drmgemshmemmmap allows users to call mmap with PROTWRITE and MAPPRIVATE flag causing a kernel panic due to BUGON in...