At the point of sysfs callback, the call to gup is
done without mmap_sem (or any lock for that matter).
This is racy. As such, use the get_user_pages_fast()
alternative and safely avoid taking the lock, if possible.
Signed-off-by: NDavidlohr Bueso <dbueso@suse.de>
Signed-off-by: NTony Luck <tony.luck@intel.com>