perf/core: Fix WARN in perf_sigtrap()

Since exit_task_work() runs after perf_event_exit_task_context() updated
ctx->task to TASK_TOMBSTONE, perf_sigtrap() from perf_pending_task() might
observe event->ctx->task == TASK_TOMBSTONE.

Swap the early exit tests in order not to hit WARN_ON_ONCE().

Closes: https://syzkaller.appspot.com/bug?extid=2fe61cb2a86066be6985


Reported-by: syzbot <syzbot+2fe61cb2a86066be6985@syzkaller.appspotmail.com>
Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/b1c224bd-97f9-462c-a3e3-125d5e19c983@I-love.SAKURA.ne.jp