e2fsck: Go back to the original superblock if the backup sb is invalid

In the case where the block group descriptors appear corrupt, e2fsck
will try to use the backup superblock.  However, it could be that the
backup superblock itself is completely corrupted, in which e2fsck
should go back to the original superblock instead of refusing to fix
the file system.

Addresses-Debian-Bug: #516820

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