posix-clocks: Introduce clock_get_ktime() callback

The callsite in common_timer_get() has already a comment:
    /*
     * The timespec64 based conversion is suboptimal, but it's not
     * worth to implement yet another callback.
     */
    kc->clock_get(timr->it_clock, &ts64);
    now = timespec64_to_ktime(ts64);

The upcoming support for time namespaces requires to have access to:

 - The time in a task's time namespace for sys_clock_gettime()
 - The time in the root name space for common_timer_get()

That adds a valid reason to finally implement a separate callback which
returns the time in ktime_t format.

Suggested-by: Thomas Gleixner <tglx@linutronix.de>
Co-developed-by: Dmitry Safonov <dima@arista.com>
Signed-off-by: Andrei Vagin <avagin@gmail.com>
Signed-off-by: Dmitry Safonov <dima@arista.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20191112012724.250792-10-dima@arista.com