drm: set FMODE_UNSIGNED_OFFSET for drm files

BugLink: https://bugs.launchpad.net/bugs/1777063

commit 76ef6b28 upstream.

Since we have the ttm and gem vma managers using a subset
of the file address space for objects, and these start at
0x100000000 they will overflow the new mmap checks.

I've checked all the mmap routines I could see for any
bad behaviour but overall most people use GEM/TTM VMA
managers even the legacy drivers have a hashtable.

Reported-and-Tested-by: Arthur Marsh (amarsh04 on #radeon)
Fixes: be83bbf8 (mmap: introduce sane default mmap limits)
Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Juerg Haefliger <juergh@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>

drivers/gpu/drm/drm_fops.c

@@ -209,6 +209,7 @@ static int drm_open_helper(struct file *filp, struct drm_minor *minor)
 		return -ENOMEM;
 
 	filp->private_data = priv;
+	filp->f_mode |= FMODE_UNSIGNED_OFFSET;
 	priv->filp = filp;
 	priv->uid = current_euid();
 	priv->pid = get_pid(task_pid(current));