Make ext2fs_check_desc() more stringent to force use of backup superbocks

Make ext2fs_check_desc() more stringent to force use of backup superbocks

E2fsck could to do more damage to a filesystem by trying to relocate
inode tables due to corrupted block group descriptors, and the
relocation could seriously damage the filesystem.

This patch enhances ext2fs_check_desk() so it detects more
self-inconsistent block group descriptors, including the cases where
e2sck might be tempted to relocate the inode table, and reports the
block group descriptors as invalid; this will cause e2fsck to attempt
to use the backup superblocks, which hopefully have not been trashed.

Addresses-Sourceforge-Bug: #1840291

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