uprobes: Fix utask->xol_vaddr leak in pre_ssout()

pre_ssout() should do xol_free_insn_slot() if arch_uprobe_pre_xol()
fails, otherwise nobody will free the allocated slot.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Anton Arapov <anton@redhat.com>
Acked-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
index 8d9c5bc..0527379 100644
--- a/kernel/events/uprobes.c
+++ b/kernel/events/uprobes.c
@@ -1306,6 +1306,7 @@
 {
 	struct uprobe_task *utask;
 	unsigned long xol_vaddr;
+	int err;
 
 	utask = current->utask;
 
@@ -1316,7 +1317,13 @@
 	utask->xol_vaddr = xol_vaddr;
 	utask->vaddr = bp_vaddr;
 
-	return arch_uprobe_pre_xol(&uprobe->arch, regs);
+	err = arch_uprobe_pre_xol(&uprobe->arch, regs);
+	if (unlikely(err)) {
+		xol_free_insn_slot(current);
+		return err;
+	}
+
+	return 0;
 }
 
 /*