Ext4Pkg: Add base containing record macro for EXT4_FILE

We shouldn't use direct casts, because in the future it could break
the code, so using BASE_CR would be safe against possible structure
changes and rearrangements

Cc: Marvin Häuser <mhaeuser@posteo.de>
Cc: Pedro Falcato <pedro.falcato@gmail.com>
Cc: Vitaly Cheptsov <vit9696@protonmail.com>
Signed-off-by: Savva Mitrofanov <savvamtr@gmail.com>
Reviewed-by: Marvin Häuser <mhaeuser@posteo.de>
Reviewed-by: Pedro Falcato <pedro.falcato@gmail.com>