perf/core: Fix cpuctx refcounting

perf/core: Fix cpuctx refcounting

commit 889c58b3155ff4c8e8671c95daef63d6fabbb6b1 upstream.

Audit of the refcounting turned up that perf_pmu_migrate_context()
fails to migrate the ctx refcount.

Fixes: bd2756811766 ("perf: Rewrite core context handling")
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lkml.kernel.org/r/20230612093539.085862001@infradead.org
Cc: <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>