time: Add struct __kernel_timex

time: Add struct __kernel_timex

struct timex uses struct timeval internally.
struct timeval is not y2038 safe.
Introduce a new UAPI type struct __kernel_timex
that is y2038 safe.

struct __kernel_timex uses a timeval type that is
similar to struct __kernel_timespec which preserves the
same structure size across 32 bit and 64 bit ABIs.
struct __kernel_timex also restructures other members of the
structure to make the structure the same on 64 bit and 32 bit
architectures.
Note that struct __kernel_timex is the same as struct timex
on a 64 bit architecture.

The above solution is similar to other new y2038 syscalls
that are being introduced: both 32 bit and 64 bit ABIs
have a common entry, and the compat entry supports the old 32 bit
syscall interface.

Alternatives considered were:
1. Add new time type to struct timex that makes use of padded
   bits. This time type could be based on the struct __kernel_timespec.
   modes will use a flag to notify which time structure should be
   used internally.
   This needs some application level changes on both 64 bit and 32 bit
   architectures. Although 64 bit machines could continue to use the
   older timeval structure without any changes.

2. Add a new u8 type to struct timex that makes use of padded bits. This
   can be used to save higher order tv_sec bits. modes will use a flag to
   notify presence of such a type.
   This will need some application level changes on 32 bit architectures.

3. Add a new compat_timex structure that differs in only the size of the
   time type; keep rest of struct timex the same.
   This requires extra syscalls to manage all 3 cases on 64 bit
   architectures. This will not need any application level changes but will
   add more complexity from kernel side.

Signed-off-by: Deepa Dinamani <deepa.kernel@gmail.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>