posix-timers: Convert clock_nanosleep_restart to clockid_to_kclock()

Use the new kclock decoding function in clock_nanosleep_restart.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: John Stultz <johnstul@us.ibm.com>
Tested-by: Richard Cochran <richard.cochran@omicron.at>
LKML-Reference: <20110201134418.131263211@linutronix.de>