fs/sysv: Use dir_put_page() in sysv_rename()

Use the dir_put_page() helper in sysv_rename() instead of open-coding two
kunmap() + put_page().

Cc: Al Viro <viro@zeniv.linux.org.uk>
Suggested-by: Ira Weiny <ira.weiny@intel.com>
Signed-off-by: Fabio M. De Francesco <fmdefrancesco@gmail.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>