Fix an inconsistency in the L2P index prefetcher code.
This does not effect correctness in any way.

* subversion/libsvn_fs_fs/index.c
  (l2p_index_lookup): Prefetch data using the same block size as
                      everywhere else.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1590460 13f79535-47bb-0310-9956-ffa450edef68