epoll: annotate racy check

[ Upstream commit 6474353a ]

Epoll relies on a racy fastpath check during __fput() in
eventpoll_release() to avoid the hit of pointlessly acquiring a
semaphore. Annotate that race by using WRITE_ONCE() and READ_ONCE().

Link: https://lore.kernel.org/r/66edfb3c.050a0220.3195df.001a.GAE@google.com
Link: https://lore.kernel.org/r/20240925-fungieren-anbauen-79b334b00542@brauner


Reviewed-by: Jan Kara <jack@suse.cz>
Reported-by:  <syzbot+3b6b32dc50537a49bb4a@syzkaller.appspotmail.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>