Ext4Pkg: Fix shift out of bounds in Ext4OpenSuperblock

Missing check for wrong s_log_block_size exponent leads to shift out of
bounds. Limit block size to 2 MiB

Cc: Marvin Häuser <mhaeuser@posteo.de>
Cc: Pedro Falcato <pedro.falcato@gmail.com>
Cc: Vitaly Cheptsov <vit9696@protonmail.com>
Fixes: d9ceedca ("Ext4Pkg: Add Ext4Dxe driver.")
Signed-off-by: Savva Mitrofanov <savvamtr@gmail.com>
Reviewed-by: Pedro Falcato <pedro.falcato@gmail.com>
Reviewed-by: Marvin Häuser <mhaeuser@posteo.de>