e2fsprogs: Disallow tune2fs enabling sparse_super with ext4 meta_bg enabled

e2fsprogs: Disallow tune2fs enabling sparse_super with ext4 meta_bg enabled

When meta_bg feature is enabled, group descriptor block is allocated
every 128 block group (or every 64 block group if 64bit feature is
enabled).

In such situation, files in block group more than #128 will be removed
if sparse_super feature is enabled with tune2fs and afterwards
necessary e2fsck running.

Because tune2fs does not reallocate group descriptor blocks but just
set sparse_super feature.  If ext4 has sparse_super,
ext2fs_descriptor_block_loc2() called by e2fsck thinks the block group
(e.g. #128) that it has group descriptor block at the head offset. But
that offset is used as backup super block before.  So e2fsck fixes
ext4 based on invalid group descriptor blocks and this cause data
lost.

The patch avoids this problem simply by disallow tune2fs enabling
sparse_super if meta_bg is enabled.

Steps to reproduce:

1. Create ext4 which has meta_bg, ^sparse_super and 129+ block groups.
	# mke2fs -t ext4 -O meta_bg,^resize_inode,^sparse_super DEV 17G
	# mount DEV /MP

2. Create direcotry and files which use block group #128's metadata.
	# echo $((8192*128+1)) > /sys/fs/ext4/DEV/inode_goal
	# mkdir /MP/DIR
	# for i in $(seq 1 100); do dd if=/dev/urandom of=/MP/DIR/file$i bs=1024 count=10; done

3. Enable sparse_super with tune2fs then execute e2fsck.
   Data in block group #128 will be lost!!
	# umount DEV
	# tune2fs -O sparse_super DEV
	# e2fsck/e2fsck -yf DEV

Signed-off-by: Akira Fujita <a-fujita@rs.jp.ne.cocm>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>