bounds: support non-power-of-two CONFIG_NR_CPUS

ilog2() rounds down, so for example when PowerPC 85xx sets CONFIG_NR_CPUS
to 24, we will only allocate 4 bits to store the number of CPUs instead of
5.  Use bits_per() instead, which rounds up.  Found by code inspection. 
The effect of this would probably be a misaccounting when doing NUMA
balancing, so to a user, it would only be a performance penalty.  The
effects may be more wide-spread; it's hard to tell.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Matthew Wilcox (Oracle) <[email protected]>
Fixes: 90572890d2 ("mm: numa: Change page last {nid,pid} into {cpu,pid}")
Reviewed-by: Rik van Riel <[email protected]>
Acked-by: Mel Gorman <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

kernel/bounds.c

@@ -19,7 +19,7 @@ int main(void)
 	DEFINE(NR_PAGEFLAGS, __NR_PAGEFLAGS);
 	DEFINE(MAX_NR_ZONES, __MAX_NR_ZONES);
 #ifdef CONFIG_SMP
-	DEFINE(NR_CPUS_BITS, ilog2(CONFIG_NR_CPUS));
+	DEFINE(NR_CPUS_BITS, bits_per(CONFIG_NR_CPUS));
 #endif
 	DEFINE(SPINLOCK_SIZE, sizeof(spinlock_t));
 #ifdef CONFIG_LRU_GEN