Ext4Pkg: Add ext2/3 support

BZ: https://bugzilla.tianocore.org/show_bug.cgi?id=3745



Adds ext2/3 support by supporting (legacy) block maps.
Also fixes a bug regarding uninitialised extents.

Cc: Leif Lindholm <leif@nuviainc.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Signed-off-by: Pedro Falcato <pedro.falcato@gmail.com>
Acked-by: Liming Gao <gaoliming@byosoft.com.cn>