e2fsck: backup superblocks if key constants have changed

e2fsck: backup superblocks if key constants have changed

If the primary superblock differs from the backup superblock in
certain key respects, force a full check (if e2fsck was invoked in
preen mode).  If the filesystem check passes cleanly, and the
filesystem was opened in read/write mode, then write the primary
superblock to all of the backups.

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