y2038: time: avoid timespec usage in settimeofday()

The compat_get_timeval() and timeval_valid() interfaces are deprecated
and getting removed along with the definition of struct timeval itself.

Change the two implementations of the settimeofday() system call to
open-code these helpers and completely avoid references to timeval.

The timeval_valid() call is not needed any more here, only a check to
avoid overflowing tv_nsec during the multiplication, as there is another
range check in do_sys_settimeofday64().

Tested-by:  <syzbot+dccce9b26ba09ca49966@syzkaller.appspotmail.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>