Stop playing silly games with the VM_ACCOUNT flag

The mmap_region() code would temporarily set the VM_ACCOUNT flag for
anonymous shared mappings just to inform shmem_zero_setup() that it
should enable accounting for the resulting shm object.  It would then
clear the flag after calling ->mmap (for the /dev/zero case) or doing
shmem_zero_setup() (for the MAP_ANON case).

This just resulted in vma merge issues, but also made for just
unnecessary confusion.  Use the already-existing VM_NORESERVE flag for
this instead, and let shmem_{zero|file}_setup() just figure it out from
that.

This also happens to make it obvious that the new DRI2 GEM layer uses a
non-reserving backing store for its object allocation - which is quite
possibly not intentional.  But since I didn't want to change semantics
in this patch, I left it alone, and just updated the caller to use the
new flag semantics.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/gpu/drm/drm_gem.c

@@ -136,7 +136,7 @@ drm_gem_object_alloc(struct drm_device *dev, size_t size)
 	obj = kcalloc(1, sizeof(*obj), GFP_KERNEL);
 
 	obj->dev = dev;
-	obj->filp = shmem_file_setup("drm mm object", size, 0);
+	obj->filp = shmem_file_setup("drm mm object", size, VM_NORESERVE);
 	if (IS_ERR(obj->filp)) {
 		kfree(obj);
 		return NULL;