kernel-rt fix: delay cleanup of utrace struct after removal from task_utrace_table

kernel-rt fix: delay cleanup of utrace struct after removal from task_utrace_table

Since utrace->lock is now an ordinary spinlock, we cannot grab it in a
(raw) task_utrace_lock section, nor can we do memory management on kernel-rt
in a raw_spinlock section. Thus, in utrace_exit(), we dump utrace structures
to a separate hlist (guarded by a separate, non-raw utrace_cleanup_lock) before
calling utrace_cleanup() to free them.

* runtime/stp_utrace.c (utrace_cleanup_list, utrace_cleanup_lock): new data structures.
(utrace_exit): dump utrace structures to utrace_cleanup_list before releasing
task_utrace_lock and then actually cleaning up.
(utrace_cleanup): now requires utrace_cleanup_lock instead of task_utrace_lock.