e2fsck: allow to fix some superblock corruptions

e2fsck: allow to fix some superblock corruptions

Add a flag to ext2fs_open() which allows to open a filesystem even if
superblock is somewhat inconsistent. Use this flag from e2fsck as a last
resort to try to fix the superblok.

Currently, the flag does nothing. We'll relax checks in ext2fs_open()
once e2fsck is able to handle corresponding corruption gracefully.

Reviewed-by: Andreas Dilger <adilger@dilger.ca>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>