time: Catch invalid timespec sleep values in __timekeeping_inject_sleeptime

Arve suggested making sure we catch possible negative sleep time
intervals that could be passed into timekeeping_inject_sleeptime.

CC: Arve Hjønnevåg <arve@android.com>
CC: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: John Stultz <john.stultz@linaro.org>