y2038: ipc: remove __kernel_time_t reference from headers

There are two structures based on time_t that conflict between libc and
kernel: timeval and timespec. Both are now renamed to __kernel_old_timeval
and __kernel_old_timespec.

For time_t, the old typedef is still __kernel_time_t. There is nothing
wrong with that name, but it would be nice to not use that going forward
as this type is used almost only in deprecated interfaces because of
the y2038 overflow.

In the IPC headers (msgbuf.h, sembuf.h, shmbuf.h), __kernel_time_t is only
used for the 64-bit variants, which are not deprecated.

Change these to a plain 'long', which is the same type as __kernel_time_t
on all 64-bit architectures anyway, to reduce the number of users of the
old type.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>