s390/hypfs: use store_tod_clock_ext()

Use store_tod_clock_ext() instead of get_tod_clock_ext().
Unfortunately one usage has to be converted to a cast, since
otherwise a uapi header file would have to be changed.

Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>