[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: NAkinobu Mita <mita@fixstars.com>
Signed-off-by: NPaul Mackerras <paulus@samba.org>

include/asm-powerpc/pgalloc-64.h

@@ -90,7 +90,8 @@ static inline pte_t *pte_alloc_one_kernel(struct mm_struct *mm,
 static inline struct page *pte_alloc_one(struct mm_struct *mm,
 					 unsigned long address)
 {
-	return virt_to_page(pte_alloc_one_kernel(mm, address));
+	pte_t *pte = pte_alloc_one_kernel(mm, address);
+	return pte ? virt_to_page(pte) : NULL;
 }
 
 static inline void pte_free_kernel(pte_t *pte)