powerpc/mm/hash: Add missing isync prior to kernel stack SLB switch

Currently we do not have an isync, or any other context synchronizing
instruction prior to the slbie/slbmte in _switch() that updates the
SLB entry for the kernel stack.

However that is not correct as outlined in the ISA.

From Power ISA Version 3.0B, Book III, Chapter 11, page 1133:

  "Changing the contents of ... the contents of SLB entries ... can
   have the side effect of altering the context in which data
   addresses and instruction addresses are interpreted, and in which
   instructions are executed and data accesses are performed.
   ...
   These side effects need not occur in program order, and therefore
   may require explicit synchronization by software.
   ...
   The synchronizing instruction before the context-altering
   instruction ensures that all instructions up to and including that
   synchronizing instruction are fetched and executed in the context
   that existed before the alteration."

And page 1136:

  "For data accesses, the context synchronizing instruction before the
   slbie, slbieg, slbia, slbmte, tlbie, or tlbiel instruction ensures
   that all preceding instructions that access data storage have
   completed to a point at which they have reported all exceptions
   they will cause."

We're not aware of any bugs caused by this, but it should be fixed
regardless.

Add the missing isync when updating kernel stack SLB entry.

Cc: stable@vger.kernel.org
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
[mpe: Flesh out change log with more ISA text & explanation]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/kernel/entry_64.S

@@ -596,6 +596,7 @@ END_MMU_FTR_SECTION_IFSET(MMU_FTR_1T_SEGMENT)
 	 * actually hit this code path.
 	 */
 
+	isync
 	slbie	r6
 	slbie	r6		/* Workaround POWER5 < DD2.1 issue */
 	slbmte	r7,r0