skd: use ktime_get_real_seconds()

Like many storage drivers, skd uses an unsigned 32-bit number for
interchanging the current time with the firmware. This will overflow in
y2106 and is otherwise safe.

However, the get_seconds() function is generally considered deprecated
since the behavior is different between 32-bit and 64-bit architectures,
and using it may indicate a bigger problem.

To annotate that we've thought about this, let's add a comment here
and migrate to the ktime_get_real_seconds() function that consistently
returns a 64-bit number.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>