powerpc/non-smp: Unconditionaly call smp_mb() on switch_mm

Commit 3ccfebed ("powerpc, membarrier: Skip memory barrier in
switch_mm()") added some logic to skip the smp_mb() in
switch_mm_irqs_off() before the call to switch_mmu_context().

However, on non SMP smp_mb() is just a compiler barrier and doing
it unconditionaly is simpler than the logic used to check whether the
barrier is needed or not.

After the patch:

00000000 <switch_mm_irqs_off>:
...
   c:	7c 04 18 40 	cmplw   r4,r3
  10:	81 24 00 24 	lwz     r9,36(r4)
  14:	91 25 04 c8 	stw     r9,1224(r5)
  18:	4d 82 00 20 	beqlr
  1c:	48 00 00 00 	b       1c <switch_mm_irqs_off+0x1c>
			1c: R_PPC_REL24	switch_mmu_context

Before the patch:

00000000 <switch_mm_irqs_off>:
...
   c:	7c 04 18 40 	cmplw   r4,r3
  10:	81 24 00 24 	lwz     r9,36(r4)
  14:	91 25 04 c8 	stw     r9,1224(r5)
  18:	4d 82 00 20 	beqlr
  1c:	81 24 00 28 	lwz     r9,40(r4)
  20:	71 29 00 0a 	andi.   r9,r9,10
  24:	40 82 00 34 	bne     58 <switch_mm_irqs_off+0x58>
  28:	48 00 00 00 	b       28 <switch_mm_irqs_off+0x28>
			28: R_PPC_REL24	switch_mmu_context
...
  58:	2c 03 00 00 	cmpwi   r3,0
  5c:	41 82 ff cc 	beq     28 <switch_mm_irqs_off+0x28>
  60:	48 00 00 00 	b       60 <switch_mm_irqs_off+0x60>
			60: R_PPC_REL24	switch_mmu_context
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/e9d501da0c59f60ca767b1b3ea4603fce6d02b9e.1625486440.git.christophe.leroy@csgroup.eu

arch/powerpc/include/asm/membarrier.h

@@ -12,7 +12,8 @@ static inline void membarrier_arch_switch_mm(struct mm_struct *prev,
 	 * when switching from userspace to kernel is not needed after
 	 * store to rq->curr.
 	 */
-	if (likely(!(atomic_read(&next->membarrier_state) &
+	if (IS_ENABLED(CONFIG_SMP) &&
+	    likely(!(atomic_read(&next->membarrier_state) &
 		     (MEMBARRIER_STATE_PRIVATE_EXPEDITED |
 		      MEMBARRIER_STATE_GLOBAL_EXPEDITED)) || !prev))
 		return;