libext2fs: allow file systems which have insane values in s_desc_size

libext2fs: allow file systems which have insane values in s_desc_size

If the 64-bit feature is not set, the kernel does not pay attention to
the s_desc_size field in the superblock.  Change ext2fs_open2() and
other library functions to similarly ignore s_desc_size if the 64-bit
feature is not set.

The EXT2_DESC_SIZE macro should be used in most cases instead of
referecing the s_desc_size field directly.

Signed-off-by: Theodore Ts'o <tytso@mit.edu>