hrtimer: Move copyout of remaining time to do_nanosleep()

The hrtimer nanosleep() implementation can be simplified by moving the copy
out of the remaining time to do_nanosleep() which is shared between the
real nanosleep function and the restart function.

The pointer to the timespec64 which is updated is already stored in the
restart block at the call site, so the seperate handling of nanosleep and
restart function can be avoided.

[ tglx: Added changelog ]

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: John Stultz <john.stultz@linaro.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20170607084241.28657-4-viro@ZenIV.linux.org.uk