s390: hypfs: use ktime_get_seconds() instead of get_seconds()

time_t and get_seconds() are deprecated because they will overflow on
32-bit architectures in the future. This is not a problem on 64-bit s390,
but we should use proper interfaces anyway.

Besides moving to the time64_t based interface, the CLOCK_MONOTONIC
based ktime_get_seconds() is preferred for kernel internal timekeeping
because it does not behave in unexpected ways during leap second changes
or settimeofday() calls.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>