Use svn_io_write_atomic() in two more places where we reimplemented this
behavior locally.

* subversion/libsvn_fs_fs/fs_fs.c
  (write_revprop_generation_file,
   write_revnum_file): Use svn_io_write_atomic() instead of local
    reimplementation.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1488137 13f79535-47bb-0310-9956-ffa450edef68