powerpc/kprobes: Fix handling of instruction emulation on probe re-entry

Commit 22d8b3de ("powerpc/kprobes: Emulate instructions on kprobe
handler re-entry") enabled emulating instructions on kprobe re-entry,
rather than single-stepping always. However, we didn't update the single
stepping code to only be run if the emulation fails. Also, we missed
re-enabling preemption if the instruction emulation was successful. Fix
those issues.

Fixes: 22d8b3de ("powerpc/kprobes: Emulate instructions on kprobe handler re-entry")
Signed-off-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/kernel/kprobes.c

@@ -305,16 +305,17 @@ int kprobe_handler(struct pt_regs *regs)
 			save_previous_kprobe(kcb);
 			set_current_kprobe(p, regs, kcb);
 			kprobes_inc_nmissed_count(p);
-			prepare_singlestep(p, regs);
 			kcb->kprobe_status = KPROBE_REENTER;
 			if (p->ainsn.boostable >= 0) {
 				ret = try_to_emulate(p, regs);
 
 				if (ret > 0) {
 					restore_previous_kprobe(kcb);
+					preempt_enable_no_resched();
 					return 1;
 				}
 			}
+			prepare_singlestep(p, regs);
 			return 1;
 		} else {
 			if (*addr != BREAKPOINT_INSTRUCTION) {