[POWERPC] Clean up access to thread_info in assembly

Use (31-THREAD_SHIFT) to get to thread_info from stack pointer.  This makes
the code a bit easier to read and more robust if we ever change THREAD_SHIFT.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>

arch/powerpc/mm/hash_low_32.S

@@ -191,7 +191,7 @@ _GLOBAL(add_hash_page)
 	add	r3,r3,r0		/* note create_hpte trims to 24 bits */
 
 #ifdef CONFIG_SMP
-	rlwinm	r8,r1,0,0,18		/* use cpu number to make tag */
+	rlwinm	r8,r1,0,0,(31-THREAD_SHIFT) /* use cpu number to make tag */
 	lwz	r8,TI_CPU(r8)		/* to go in mmu_hash_lock */
 	oris	r8,r8,12
 #endif /* CONFIG_SMP */
@@ -526,7 +526,7 @@ _GLOBAL(flush_hash_pages)
 #ifdef CONFIG_SMP
 	addis	r9,r7,mmu_hash_lock@ha
 	addi	r9,r9,mmu_hash_lock@l
-	rlwinm	r8,r1,0,0,18
+	rlwinm	r8,r1,0,0,(31-THREAD_SHIFT)
 	add	r8,r8,r7
 	lwz	r8,TI_CPU(r8)
 	oris	r8,r8,9