powerpc/powernv: Fix SMT4 forcing idle code

The PSSCR value is not stored to PACA_REQ_PSSCR if the CPU does not
have the XER[SO] bug.

Fix this by storing up-front, outside the workaround code. The initial
test is not required because it is a slow path.

The workaround is made to depend on CONFIG_KVM_BOOK3S_HV_POSSIBLE, to
match pnv_power9_force_smt4_catch() where it is used. Drop the comment
on pnv_power9_force_smt4_catch() as it's no longer true.

Fixes: 7672691a08 ("powerpc/powernv: Provide a way to force a core into SMT4 mode")
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/platforms/powernv/idle.c

@@ -397,10 +397,6 @@ void power9_idle(void)
  * all other threads not to stop, and sending a message to any
  * that are in a stop state.
  * Must be called with preemption disabled.
- *
- * DO NOT call this unless cpu_has_feature(CPU_FTR_P9_TM_XER_SO_BUG) is
- * true; otherwise this function will hang the system, due to the
- * optimization in power9_idle_stop.
  */
 void pnv_power9_force_smt4_catch(void)
 {