libext2fs: correctly write up the backup superblocks in big endian systems

libext2fs: correctly write up the backup superblocks in big endian systems

This bug has been around since we added support for metadata
checksums, but it was unmasked by commit bf9f3b6d5b ("e2fsck: exit
with exit status 0 if no errors were fixed").  The backup superblocks
are not supposed to have the EXT2_VALID_FS or the NEEDS_RECOVERY bits
set, and earlier 1.43.x versions of e2fsprogs were byte swapping the
shadow superblock each time it was written, so that every other backup
superblock was incorrectly byte swapped.

Fortunately the primary backup superblock was correctly written
(modulo having the VALID_FS bit set when it should not have been set)
so for the most part no one noticed.  And very few architectures use
big endian byte ordering these days.  (Even IBM has seen the light
with the ppcle architecture.  :-)

Fortunately commit bf9f3b6d5b caused f_desc_size_bad and
f_resize_inode to fail on a big endian system, which allowed me to
notice the issue and investigate.

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