posix-cpu-timers: Move copyout of timespec into do_cpu_nanosleep()

The posix-cpu-timer nanosleep() implementation can be simplified by moving
the copy out of the remaining time to do_cpu_nanosleep() which is shared
between the real nanosleep function and the restart function.

The pointer to the timespec64 which is updated has to be stored in the
restart block anyway. Instead of storing it only in the restart case, store
it before calling do_cpu_nanosleep() and copy the remaining time in the
signal exit path.

[ 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/20170607084241.28657-1-viro@ZenIV.linux.org.uk