Handle extents getting added before an indirect block.

Handle extents getting added before an indirect block.

Now that indirect blocks are 'mobile' and can have a
start address, it is possible that an extent in uninc_table
starts before the indirect addresses.
In this case the logic that assured us there was room for all
the addresses fall down.

So detect that case earlier and force the indirect block
to either start at the start of the uninc bloc, or become
an extent block.  Possibly it will split as part of this.

Signed-off-by: NeilBrown <neilb@suse.de>