Don't update ret until fixed file is closed, otherwise the file slot
becomes the error code.

Fixes: a7c41b46 ("io_uring: let IORING_OP_FILES_UPDATE support choosing fixed file slots")
Signed-off-by: Hao Xu <howeyxu@tencent.com>
[pavel: 5.19 rebase]
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>