perf: Rename perf_event_exit_task(.child)

The task passed to perf_event_exit_task() is not a child, it is
current. Fix this confusing naming, since much of the rest of the code
also relies on it being current.

Specifically, both exec() and exit() callers use it with current as
the argument.

Notably, task_ctx_sched_out() doesn't make much sense outside of
current.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Ravi Bangoria <ravi.bangoria@amd.com>
Link: https://lkml.kernel.org/r/20250307193305.486326750@infradead.org