timekeeping: Minor fixup for timespec64->timespec assignment

In the GENERIC_TIME_VSYSCALL_OLD update_vsyscall implementation,
we take the tk_xtime() value, which returns a timespec64, and
store it in a timespec.

This luckily is ok, since the only architectures that use
GENERIC_TIME_VSYSCALL_OLD are ia64 and ppc64, which are both
64 bit systems where timespec64 is the same as a timespec.

Even so, for cleanliness reasons, use the conversion function
to assign the proper type.
Signed-off-by: NJohn Stultz <john.stultz@linaro.org>

kernel/time/timekeeping.c

@@ -338,7 +338,7 @@ static inline void update_vsyscall(struct timekeeper *tk)
 {
 	struct timespec xt;
 
-	xt = tk_xtime(tk);
+	xt = timespec64_to_timespec(tk_xtime(tk));
 	update_vsyscall_old(&xt, &tk->wall_to_monotonic, tk->tkr.clock, tk->tkr.mult,
 			    tk->tkr.cycle_last);
 }