io_uring: fix fget leak when fs don't support nowait buffered read

Heming reported a BUG when using io_uring doing link-cp on ocfs2. [1]

Do the following steps can reproduce this BUG:
mount -t ocfs2 /dev/vdc /mnt/ocfs2
cp testfile /mnt/ocfs2/
./link-cp /mnt/ocfs2/testfile /mnt/ocfs2/testfile.1
umount /mnt/ocfs2

Then umount will fail, and it outputs:
umount: /mnt/ocfs2: target is busy.

While tracing umount, it blames mnt_get_count() not return as expected.
Do a deep investigation for fget()/fput() on related code flow, I've
finally found that fget() leaks since ocfs2 doesn't support nowait
buffered read.

io_issue_sqe
|-io_assign_file  // do fget() first
  |-io_read
  |-io_iter_do_read
    |-ocfs2_file_read_iter  // return -EOPNOTSUPP
  |-kiocb_done
    |-io_rw_done
      |-__io_complete_rw_common  // set REQ_F_REISSUE
    |-io_resubmit_prep
      |-io_req_prep_async  // override req->file, leak happens

This was introduced by commit a196c78b in v5.18. Fix it by don't
re-assign req->file if it has already been assigned.

[1] https://lore.kernel.org/ocfs2-devel/ab580a75-91c8-d68a-3455-40361be1bfa8@linux.alibaba.com/T/#t



Fixes: a196c78b ("io_uring: assign non-fixed early for async work")
Cc: <stable@vger.kernel.org>
Reported-by: Heming Zhao <heming.zhao@suse.com>
Signed-off-by: Joseph Qi <joseph.qi@linux.alibaba.com>
Cc: Xiaoguang Wang <xiaoguang.wang@linux.alibaba.com>
Link: https://lore.kernel.org/r/20230228045459.13524-1-joseph.qi@linux.alibaba.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>