page-writeback.c:(.text+0x6446): undefined reference to `____ilog2_NaN'

writeback-dirty-throttling-v7

From <http://lkml.org/lkml/2011/4/16/153>

> mm/built-in.o: In function `calc_period_shift.part.10':
> page-writeback.c:(.text+0x6446): undefined reference to `____ilog2_NaN'

I cannot reproduce this error. In the git tree, calc_period_shift() is
actually quite simple:

static int calc_period_shift(void)                
{                                                 
-        return 2 + ilog2(default_backing_dev_info.avg_write_bandwidth);
+        return 2 + __ilog2_u32(default_backing_dev_info.avg_write_bandwidth);
}
where avg_write_bandwidth is of type "unsigned long".