resize2fs: do not clear resize inode for 0 resvd blocks

resize2fs: do not clear resize inode for 0 resvd blocks

I ran into odd behavior where mkfs.ext4 of a 16T filesystem would
create a resize inode with 0 reserved blocks, and mark the resize_inode
feature.

A subsequent slight downward resize of the filesystem would remove
the resize inode, making any further offline resizing impossible.

This is especially odd in light of the fact that a large downward
resize (say, to 8T) will actually add blocks to the resize inode -
so a small resize removes it, a large resize expands it ...

commit 8ade268cf2fde8629b51bfd1c044a83db88234cd had added this:

  If the filesystem is grown to the point where the resize_inode is no
  longer needed, clean it up properly so e2fsck doesn't have to.

but, it seems e2fsck does not care about this situation, either.

So, simply leave the resize_inode intact in this case, and everything
seems to be happy.

Note, this is for the 1.41.xx branch.

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