sched_waitpid.c: HIS_metric_violation: CCM

This commit refactors nxsched_waitpid() by extracting the core waiting logic
into a separate waittcb() helper function. This change significantly reduces
the cyclomatic complexity of the main function while preserving all original
functionality and improving code maintainability with better separation of
concerns between initialization and wait handling logic.

Signed-off-by: hujun5 <hujun5@xiaomi.com>

Author: hujun260