perf: Fix use-after-free in error path

The syscall error path has a use-after-free; put_pmu_ctx() will
reference ctx, therefore we must ensure ctx is destroyed after pmu_ctx
is.

Fixes: bd275681

 ("perf: Rewrite core context handling")
Reported-by:  <syzbot+b8e8c01c8ade4fe6e48f@syzkaller.appspotmail.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Chengming Zhou <zhouchengming@bytedance.com>
Link: https://lkml.kernel.org/r/Y6B3xEgkbmFUCeni@hirez.programming.kicks-ass.net