Revert "Fix implicit DeviceMapper selection"

This reverts commit 0a376291b2213699f986a7bca1cc8c4f4ed00f8d.

Signed-off-by: David Calavera <david.calavera@gmail.com>

man/docker.1.md

@@ -451,6 +451,45 @@ removed.
 
 Example use: `docker -d --storage-opt dm.blkdiscard=false`
 
+#### dm.override_udev_sync_check
+
+By default, the devicemapper backend attempts to synchronize with the
+`udev` device manager for the Linux kernel.  This option allows
+disabling that synchronization, to continue even though the
+configuration may be buggy.
+
+To view the `udev` sync support of a Docker daemon that is using the
+`devicemapper` driver, run:
+
+        $ docker info
+	[...]
+	 Udev Sync Supported: true
+	[...]
+
+When `udev` sync support is `true`, then `devicemapper` and `udev` can
+coordinate the activation and deactivation of devices for containers.
+
+When `udev` sync support is `false`, a race condition occurs between
+the`devicemapper` and `udev` during create and cleanup. The race
+condition results in errors and failures. (For information on these
+failures, see
+[docker#4036](https://github.com/docker/docker/issues/4036))
+
+To allow the `docker` daemon to start, regardless of whether `udev` sync is
+`false`, set `dm.override_udev_sync_check` to true:
+
+        $ docker -d --storage-opt dm.override_udev_sync_check=true
+
+When this value is `true`, the driver continues and simply warns you
+the errors are happening.
+
+**Note**: The ideal is to pursue a `docker` daemon and environment
+that does support synchronizing with `udev`. For further discussion on
+this topic, see
+[docker#4036](https://github.com/docker/docker/issues/4036).
+Otherwise, set this flag for migrating existing Docker daemons to a
+daemon with a supported environment.
+
 # EXEC DRIVER OPTIONS
 
 Use the **--exec-opt** flags to specify options to the exec-driver. The only