Fix cv_timedwait_hires

The user space implementation of cv_timedwait_hires() was always passing
a relative time to pthread_cond_timedwait() when an absolute time is
expected. This was accidentally introduced in commit 206971d2.

Replace two magic values with their corresponding preprocessor macro.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Richard Yao <ryao@gentoo.org>
Closes #5024