uprobes/x86: Fix arch_uprobe_disable_step() && UTASK_SSTEP_TRAPPED interaction

arch_uprobe_disable_step() should also take UTASK_SSTEP_TRAPPED into
account. In this case the probed insn was not executed, we need to
clear X86_EFLAGS_TF if it was set by us and that is all.

Again, this code will look more clean when we move it into
arch_uprobe_post_xol() and arch_uprobe_abort_xol().
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>

arch/x86/kernel/uprobes.c

@@ -706,14 +706,20 @@ void arch_uprobe_disable_step(struct arch_uprobe *auprobe)
 {
 	struct task_struct *task = current;
 	struct arch_uprobe_task	*autask	= &task->utask->autask;
+	bool trapped = (task->utask->state == UTASK_SSTEP_TRAPPED);
 	struct pt_regs *regs = task_pt_regs(task);
 	/*
 	 * The state of TIF_BLOCKSTEP was not saved so we can get an extra
 	 * SIGTRAP if we do not clear TF. We need to examine the opcode to
 	 * make it right.
 	 */
+	if (unlikely(trapped)) {
+		if (!autask->saved_tf)
+			regs->flags &= ~X86_EFLAGS_TF;
+	} else {
 		if (autask->saved_tf)
 			send_sig(SIGTRAP, task, 0);
 		else if (!(auprobe->fixups & UPROBE_FIX_SETF))
 			regs->flags &= ~X86_EFLAGS_TF;
+	}
 }