perf: Simplify perf_event_free_task() wait

Simplify the code by moving the duplicated wakeup condition into
put_ctx().

Notably, wait_var_event() is in perf_event_free_task() and will have
set ctx->task = TASK_TOMBSTONE.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Ravi Bangoria <ravi.bangoria@amd.com>
Link: https://lkml.kernel.org/r/20250307193723.044499344@infradead.org