e2fsck: report only one sb corruption

e2fsck: report only one sb corruption

check_super_value() does not terminate in case of error anymore since
c8b20b40ebf0 "misc: add plausibility checks to
debugfs/tune2fs/dumpe2fs/e2fsck" which removed the PR_FATAL flag from
PR_0_SB_CORRUPT problem. This results in potentially many errors for
superblock being printed including the long message about how to deal
with corrupted superblock. Restore the original behavior of reporting
only one error and also remove the comments 'never get here' as they are
not true anymore.

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