[POWERPC] Fix gettimeofday inaccuracies

There are two problems in the powerpc gettimeofday code which can
cause incorrect results to be returned.

The first is that there is a race between do_gettimeofday and the
timer interrupt:

1. do_gettimeofday does get_tb()

2. decrementer exception on boot cpu which runs timer_recalc_offset,
   which also samples the timebase and updates the do_gtod structure
   with a greater timebase value.

3. do_gettimeofday calls __do_gettimeofday, which leads to the
   negative result from tb_val - temp_varp->tb_orig_stamp.

The second is caused by taking the boot cpu offline, which can cause
the value of tb_last_jiffy to be increased past the currently
available timebase, causing the same underflow as above.

[paulus@samba.org - define and use data_barrier() instead of mb().]

Signed-off-by: Nathan Lynch <ntl@pobox.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>