[Btrfs-devel] fix block readahead in btrfs_next_leaf

[Chris Mason]

It is hard to comment on this patch because there's no description for
why you think it fixes block readahead.  But, reada_for_search tries to
cluster blocks close to the one indicated in path->nodes[level][slot],
so always using slot=zero should make us cluster by the first block
pointer in the node.

The defrag code tries to keep the nodes mostly contiguous, but it
should be better to cluster by the block we're actually looking for.

-chris