resize2fs: fix minimum size calculations

resize2fs: 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?

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>