sched: Remove needless round trip nsecs <-> tick conversion of steal time

When update_rq_clock_task() accounts the pending steal time for a task,
it converts the steal delta from nsecs to tick then from tick to nsecs.

There is no apparent good reason for doing that though because both
the task clock and the prev steal delta are u64 and store values
in nsecs.

So lets remove the needless conversion.

Cc: Ingo Molnar <mingo@kernel.org>
Cc: Marcelo Tosatti <mtosatti@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Rik van Riel <riel@redhat.com>
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>