fix minimum size calculations

The extra padding added to the minimum size calculations:

        /*
         * We need to reserve a few extra blocks if extents are
         * enabled, in case we need to grow the extent tree.  The more
         * we shrink the file system, the more space we need.
         */
        if (fs->super->s_feature_incompat & EXT3_FEATURE_INCOMPAT_EXTENTS)
                blks_needed += (fs->super->s_blocks_count - blks_needed)/500;

can go quite wrong if we've already added up more "blks_needed" than our current size, and the above subtraction wraps. This can easily happen for a filesystem which is almost completely full.

In this case, just return the current fs size as the minimum and be done with it.

With this fix we could probably call calculate_minimum_resize_size() for each resize2fs invocation and refuse to resize smaller than that?

---

Index: e2fsprogs/resize/resize2fs.c
===================================================================
@@ -2003,6 +2003,12 @@ blk_t calculate_minimum_resize_size(ext2
 	blks_needed += overhead;
 
 	/*
+	 * If at this point we've already added up more "needed" than
+	 * the current size, just return current size as minimum.
+	 */
+	if (blks_needed >= fs->super->s_blocks_count)
+		return fs->super->s_blocks_count;
+	/*
 	 * We need to reserve a few extra blocks if extents are
 	 * enabled, in case we need to grow the extent tree.  The more
 	 * we shrink the file system, the more space we need.