Ext4Pkg: Fix build on MSVC

Accessing array using index of uint64 type makes MSVC compiler to
include `__allmul` function in NOOPT which is not referenced in IA32.
So we null-terminate the string using ReadSize, which should be equal to
SymlinkSizeTmp after correct reading. Also add missing MultU64x32
in Ext4Read.

Cc: Marvin Häuser <mhaeuser@posteo.de>
Cc: Pedro Falcato <pedro.falcato@gmail.com>
Cc: Vitaly Cheptsov <vit9696@protonmail.com>
Fixes: 7c46116b ("Ext4Pkg: Add ext2/3 support")
Fixes: e81432fb ("Ext4Pkg: Add symbolic links support")
Signed-off-by: Savva Mitrofanov <savvamtr@gmail.com>
Reviewed-by: Marvin Häuser <mhaeuser@posteo.de>
Reviewed-by: Pedro Falcato <pedro.falcato@gmail.com>