Follow up to r1622926: Use the correct canonicalization check for FS paths.

* subversion/libsvn_fs_fs/low_level.c
  (read_change, svn_fs_fs__read_noderev): Use svn_fspath__is_canonical
   instead of the (incorrect, platform-specific) svn_dirent_is_canonical.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1622987 13f79535-47bb-0310-9956-ffa450edef68