libgfs2: Use the correct parent for rgrp tree insertion
lgfs2_rgrps_append() was using the parent of the last rgrp instead of
the last rgrp itself as the intended parent when inserting new rgrps
into the tree. This left the tree unbalanced in such a way as to make
lookups of the last node in the tree effectively a linear list search.
This was done for each rgrp insertion so, although it didn't cause a
noticeable slow-down for smaller file sytems, tests on 250TB volumes
requiring over a million resource groups would become CPU bound and
could take hours to complete.
This patch fixes lgfs2_rgrps_append() to use the correct parent node for
rgrp tree insertions, giving a significant performance improvement when
creating file systems on large volumes.
Resolves: rhbz#1194446
Signed-off-by: Andrew Price <anprice@redhat.com>