posix-timers: Convert clock_gettime() to clockid_to_kclock()

Use the new kclock decoding mechanism and rename the misnomed
common_clock_get() to posix_clock_realtime_get().

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: <20110201134418.611097203@linutronix.de>