Rearrange the ste_allocate()/__ste_allocate() path in the PPC64
segment table code more sensibly.  This moves various valid address
checks into the lower-level __ste_allocate(), meaning the checks don't
need to be duplicated in preload_stab().
Signed-off-by: David Gibson <dwg@au1.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>