e2fsck discard errors

[Phillip Susi <psusi@ubuntu.com>]

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I was experimenting with e2fsck -E discard on image files and found
some errors introduced by commit efa1a355.  The following section of
code attempts to discard unused portions of the inode table:


/* Discard inode table */
if (ctx->options & E2F_OPT_DISCARD) {
        blk64_t used_blks, blk, num;

        used_blks = DIV_ROUND_UP(
                (EXT2_INODES_PER_GROUP(fs->super) -
                 group_free),
                EXT2_INODES_PER_BLOCK(fs->super));

        blk = ext2fs_inode_table_loc(fs, group) +
                used_blks;
        num = fs->inode_blocks_per_group -
                used_blks;
        e2fsck_discard_blocks(ctx, manager, blk, num);
}

There are two problems with this.  The first is that trying to discard
a count of zero blocks results in an -EINVAL, which silently halts (
shouldn't this at least issue a warning? ) further discard attempts.
The second I noticed after fixing that problem and had a bunch of
valid inodes discarded.  It looks like the intent of this code is to
free the part of the inode table that lies beyond the highest used
inode, but instead it is using the count of free inodes, so when you
have some free inodes followed by some used inodes, the used inodes at
the end get discarded.

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