e2fsck: If the superblock is corrupt, automatically retry with the backup sb

e2fsck: If the superblock is corrupt, automatically retry with the backup sb

E2fsck currently only retries with the backup superblock if the
primary superblock is missing (e.g., overwritten with garbage).  If
the superblock is just corrupted enough that it looks like ext2/3/4
superblock, but it is corrupt enough that ext2fs_open2() returns an
error, e2fsck stops without retrying.  Let's fix this oversight.

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