ext2 places non-directory objects into the same blockgroup as their
directory, as long as that directory has free inodes.  It does this
even if there are no free blocks in that blockgroup (!).

This means that if there are lots of files being created at a common
point in the tree, they _all_ have the same starting blockgroup.  For
each file we do a big search forwards for the first block and the
allocations end up getting intermingled.

So this patch will avoid placing new inodes in block groups which have
no free blocks.

So far so good.  But this means that if a lot of new files are being
created under a directory (or multiple directories) which are in the
same blockgroup, all the new inodes will overflow into the same
blockgroup.  No improvement at all.

So the patch arranges for the new inode locations to be "spread out"
across different blockgroups if they are not going to be placed in
their directory's block group.  This is done by adding parent->i_ino
into the starting point for the quadratic hash.  i_ino was chosen so
that files which are in the same directory will tend to all land in the
same new blockgroup.