Ext4Pkg: Redirect internal Open("..", /) to the proper "/"

We have been hitting many issues with ".." ASSERTing on / in internal
code, so make sure that we direct /.. to /. This is safe.

Signed-off-by: Pedro Falcato <pedro.falcato@gmail.com>
Cc: Marvin Häuser <mhaeuser@posteo.de>
Reported-by: Savva Mitrofanov <savvamtr@gmail.com>
Reviewed-by: Marvin Häuser <mhaeuser@posteo.de>