Fix FSFS f7 low level checksum verification logic.

* subversion/libsvn_fs_fs/verify.c
  (compare_p2l_to_rev): Make sure start reading the items from the
                        correct file offsets.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1631550 13f79535-47bb-0310-9956-ffa450edef68