Mount without argument always print a "none" as device name mounted
because entry->parent_device is always NULL.

The problem is the mount function in fs/fs.c. parent_device is
initialized to NULL and never updated. With this patch,
parent_device is set with the mounted device name.

Moreover, the mount function has been modified to print the device
name plus device id using the dev_name function.

Signed-off-by: Franck Jullien <franck.jullien@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>