vdso vgetns() didn't mask the time source offset calculation, which
could lead to time problems with 32bit HPET.  Add the masking.

Thanks to Chuck Ebbert for tracking this down.
Signed-off-by: Andi Kleen <ak@suse.de>
Cc: Chuck Ebbert <cebbert@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>