Ext4Pkg: Fix incorrect usage of Ext4InitExtentsMap.

Fixes bug triggered by ShellPkg code, in usage of EFI_FILE_PROTOCOL's
Open().

Cc: Leif Lindholm <leif@nuviainc.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Bret Barkelew <Bret.Barkelew@microsoft.com>
Signed-off-by: Pedro Falcato <pedro.falcato@gmail.com>
Reviewed-by: Michael D Kinney <michael.d.kinney@intel.com>