tune2fs: fix memory write overflow

tune2fs: fix memory write overflow

If we apply this patch 'e2fsprogs/tune2fs: rewrite metadata checksums
when resizing inode size', we will trigger a segfault, this is because
of the inode cache issues.

Firstly we should notice that in expand_inode_table(), we have change
the super block's s_inode_size to new inode size(for example, 256).

Then we re-compute metadata checksums, see below code flow:
|-->rewrite_metadata_checksums
|----->rewrite_inodes
|-------->ext2fs_write_inode_full
In ext2fs_write_inode_full(), if an inode cache is hit, the below code will be executed:
	/* Check to see if the inode cache needs to be updated */
	if (fs->icache) {
		for (i=0; i < fs->icache->cache_size; i++) {
			if (fs->icache->cache[i].ino == ino) {
				memcpy(fs->icache->cache[i].inode, inode,
				       (bufsize > length) ? length : bufsize);
				break;
			}
		}
	}

Before executing rewrite_inodes(), actually the inode in inode cache
is allocated by old inode size(for example, 128), but here the memcpy
will obviously write overflow, '(bufsize > length) ? length : bufsize'
here will return 256(new inode size), so this is wrong, we need to fix
this.  I think we should call ext2fs_free_inode_cache() in
expand_inode_table(), to drop the inode cache, because inode size has
changed, if necessary, we will re-create this inode cache.

Steps to reproduce this bug (apply 'tune2fs: rewrite metadata checksums
when resizing inode size' first):
	dd if=/dev/zero of=file.img bs=1M count=128
	device_name=$(/sbin/losetup -f)
	/sbin/losetup -f file.img
	mkfs.ext4 -I 128 -O ^flex_bg $device_name
	tune2fs -I 256 $device_name

Signed-off-by: Xiaoguang Wang <wangxg.fnst@cn.fujitsu.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>