x86_64: Add missing mask operation to vdso

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>