sched: Prevent compiler from optimising the sched_avg_update() loop

sched: Prevent compiler from optimising the sched_avg_update() loop

GCC 4.4.1 on ARM has been observed to replace the while loop in
sched_avg_update with a call to uldivmod, resulting in the
following build failure at link-time:

kernel/built-in.o: In function `sched_avg_update':
 kernel/sched.c:1261: undefined reference to `__aeabi_uldivmod'
 kernel/sched.c:1261: undefined reference to `__aeabi_uldivmod'
make: *** [.tmp_vmlinux1] Error 1

This patch introduces a fake data hazard to the loop body to
prevent the compiler optimising the loop away.

Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Russell King <rmk@arm.linux.org.uk>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: <stable@kernel.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>