drm: set FMODE_UNSIGNED_OFFSET for drm files

commit 76ef6b28ea4f81c3d511866a9b31392caa833126 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: be83bbf8068 (mmap: introduce sane default mmap limits)
Signed-off-by: Dave Airlie <airlied@redhat.com>
[bwh: Backported to 3.16: adjust filename]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

drivers/gpu/drm/drm_fops.c

@@ -267,6 +267,7 @@ static int drm_open_helper(struct inode *inode, struct file *filp,
 		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));