xen/x86: fix for special behavior of first sys_settimeofday(NULL, &tz) invocation

xen/x86: fix for special behavior of first sys_settimeofday(NULL, &tz) invocation

The data Xen's time implementation maintains to make do_gettimeofday()
return values monotonic needs to be reset not only during normal
do_gettimeofday() invocations, but also when the clock gets warped
due to the hardware (CMOS) clock running on local (rather than UTC)
time.

Additionally there was a time window in do_gettimeofday() (between
the end of the xtime read loop and the acquiring of the monotonicity
data lock) where, if on another processor do_settimeofday() would
execute to completion, the zeroes written by the latter could get
overwritten by the former with values obtained before the time was
updated. This now gets prevented by maintaining a version for the
monotonicity data.

Signed-off-by: Jan Beulich <jbeulich@novell.com>