io_uring: use bottom half safe lock for fixed file data

io_file_data_ref_zero() can be invoked from soft-irq from the RCU core,
hence we need to ensure that the file_data lock is bottom half safe. Use
the _bh() variants when grabbing this lock.

Reported-by:  <syzbot+1f4ba1e5520762c523c6@syzkaller.appspotmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>