preload_stab() is only ever called (once) from flush_stab(), and
flush_stab() is only ever called from switch_mm().  So, combine both
functions into the more accurately named switch_stab(), called from
switch_mm().
Signed-off-by: David Gibson <dwg@au1.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>