dm mpath: fix infinite recursion in ioctl when no paths and !queue_if_no_path

In multipath_prepare_ioctl(),
  - pgpath is a path selected from available paths
  - m->queue_io is true if we cannot send a request immediately to
    paths, either because:
      * there is no available path
      * the path group needs activation (pg_init)
          - pg_init is not started
          - pg_init is still running
  - m->queue_if_no_path is true if the device is configured to queue
    I/O if there are no available paths

If !pgpath && !m->queue_if_no_path, the handler should return -EIO.
However in the course of refactoring the condition check has broken
and returns success in that case.  Since bdev points to the dm device
itself, dm_blk_ioctl() calls __blk_dev_driver_ioctl() for itself and
recurses until crash.

You could reproduce the problem like this:

  # dmsetup create mp --table '0 1024 multipath 0 0 0 0'
  # sg_inq /dev/mapper/mp
  <crash>
  [  172.648615] BUG: unable to handle kernel paging request at fffffffc81b10268
  [  172.662843] PGD 19dd067 PUD 0
  [  172.666269] Thread overran stack, or stack corrupted
  [  172.671808] Oops: 0000 [#1] SMP
  ...

Fix the condition check with some clarifications.

Fixes: e56f81e0b01e ("dm: refactor ioctl handling")
Signed-off-by: Jun'ichi Nomura <j-nomura@ce.jp.nec.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/dm-mpath.c

@@ -1537,29 +1537,31 @@ static int multipath_prepare_ioctl(struct dm_target *ti,
 		struct block_device **bdev, fmode_t *mode)
 {
 	struct multipath *m = ti->private;
-	struct pgpath *pgpath;
 	unsigned long flags;
 	int r;
 
-	r = 0;
-
 	spin_lock_irqsave(&m->lock, flags);
 
 	if (!m->current_pgpath)
 		__choose_pgpath(m, 0);
 
-	pgpath = m->current_pgpath;
-
-	if (pgpath) {
-		*bdev = pgpath->path.dev->bdev;
-		*mode = pgpath->path.dev->mode;
+	if (m->current_pgpath) {
+		if (!m->queue_io) {
+			*bdev = m->current_pgpath->path.dev->bdev;
+			*mode = m->current_pgpath->path.dev->mode;
+			r = 0;
+		} else {
+			/* pg_init has not started or completed */
+			r = -ENOTCONN;
+		}
+	} else {
+		/* No path is available */
+		if (m->queue_if_no_path)
+			r = -ENOTCONN;
+		else
+			r = -EIO;
 	}
 
-	if ((pgpath && m->queue_io) || (!pgpath && m->queue_if_no_path))
-		r = -ENOTCONN;
-	else if (!*bdev)
-		r = -EIO;
-
 	spin_unlock_irqrestore(&m->lock, flags);
 
 	if (r == -ENOTCONN) {