every struct fs_device_d contains a struct mtab_entry, so they
have a 1:1 relationship. Instead of having to use container_of
to get from a struct mtab_entry to a struct fs_device_d we can
better embed the members of struct mtab_entry into struct fs_device_d
directly.
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>