libext2fs: avoid 32-bit overflow in ext2fs_initialize with a 512M cluster size

libext2fs: avoid 32-bit overflow in ext2fs_initialize with a 512M cluster size

If the user attemps to create a 512MB cluster, we need to adjust the
defaults to avoid a 32-bit overflow of s_blocks_per_group.  Also check
to make sure that the caller of ext2fs_initialize() has not given a
value of s_clusters_per_group that would result in an overflow of
s_blocks_per_group.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Reviewed-by: Zheng Liu <wenqing.lz@taobao.com>