From: john stultz <johnstul@us.ibm.com>

Since jiffies didn't necessarily start incrementing at a second boundary,
jiffies/HZ doesn't increment at the same moment as xtime.tv_sec.  This
causes one second wobbles in the calculation of btime (xtime.tv_sec -
jiffies/HZ).

This fix increases the precision of the calculation so the usec component
of xtime is used as well.  Additionally it fixes some of the non-atomic
reading of time values.