[PATCH] s390: next_timer_interrupt overflow in stop_hz_timer

The 32 bit unsigned substraction (next - jiffies) in stop_hz_timer can
overflow if jiffies gets advanced between next_timer_interrupt and the read
under the xtime lock.  The cast to a u64 then results in a large value
which causes the cpu to wait too long.  Fix this by casting next and
jiffies independently to u64 before subtracting them.

(Spotted by Zachary Amsden <zach@vmware.com>)

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>