drm/gem-shmem: Don't store mmap'ed buffers in core dumps

Set the VM_DONTDUMP flag on mmap'ed VMAs to omit them from core
dumps. It's display-buffer memory; who knows what secrets these
buffers contain.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Chia-I Wu <olvaffe@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220209155634.3994-3-tzimmermann@suse.de