get_fs_device_by_path returns a struct device_d instead of what the
name suggests a struct fs_device_d. Also it returns the rootpath
of the corresponding fs_device. This patch changes the name of
this function to get_fs_device_and_root_path to better reflect what
the function does and changes the return type to struct fs_device_d.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>