um: fix global timer issue when using CONFIG_NO_HZ

This fixes a issue which was introduced by fe2cc53e ("uml: track and make
up lost ticks").

timeval_to_ns() returns long long and not int.  Due to that UML's timer
did not work properlt and caused timer freezes.

Signed-off-by: Richard Weinberger <richard@nod.at>
Acked-by: Pekka Enberg <penberg@kernel.org>
Cc: Jeff Dike <jdike@addtoit.com>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>