io_uring: kbuf: fix bug of not consuming ring buffer in partial io case

When we use ring-mapped provided buffer, we should consume it before
arm poll if partial io has been done. Otherwise the buffer may be used
by other requests and thus we lost the data.

Fixes: c7fb1942 ("io_uring: add support for ring mapped supplied buffers")
Signed-off-by: Hao Xu <howeyxu@tencent.com>
[pavel: 5.19 rebase]
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>