endian-annotate most on-disk structures

endian-annotate most on-disk structures

This annotates most on-disk structures for endianness;
however it does not annotate some, like the superblock, inodes,
mmp, etc, as these are swapped in-place at this point.  This is
a little inconsistent, but should help catch some endian mistakes,
at least.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>