repair: use a btree instead of a radix tree for the prefetch queue

repair: use a btree instead of a radix tree for the prefetch queue

Currently the prefetch queue in xfs_repair uses a radix tree implementation
derived from the Linux kernel one to manage it's prefetch queue.

The radix tree implement is not very memory efficient for sparse indices,
so replace it with a btree implementation that is much more efficient.
This is not that important for the prefetch queue but will be very important
for the next memory optimization patches which need a tree to store things
like the block map which are very sparse, and we do not want to deal with
two tree implementations (or rather three given that we still have avl.c
around)

Signed-off-by: Barry Naujok <bnaujok@sgi.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Alex Elder <aelder@sgi.com>