dm table: check that a dm device doesn't reference itself

If a DM device's table references itself, it will crash the kernel with an
infinite recursion.  Check for a self-reference in dm_get_device(). This
is a quick check, but it won't catch more complicated circular references.
Signed-off-by: Benjamin Marzinski <bmarzins@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>

drivers/md/dm-table.c

@@ -364,6 +364,8 @@ int dm_get_device(struct dm_target *ti, const char *path, fmode_t mode,
 		if (!dev)
 			return -ENODEV;
 	}
+	if (dev == disk_devt(t->md->disk))
+		return -EINVAL;
 
 	dd = find_device(&t->devices, dev);
 	if (!dd) {