[POWERPC] Fix return from pte_alloc_one() in out-of-memory case

pte_alloc_one() is expected to return NULL if out of memory.
But it returns virt_to_page(NULL), which is not NULL.
This fixes it.

Cc: Paul Mackerras <paulus@samba.org>
Signed-off-by: Akinobu Mita <mita@fixstars.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>