During some testing, it became evident that using IORING_OP_WRITE doesn't
hash buffered writes like the other writes commands do. That's simply
an oversight, and can cause performance regressions when doing buffered
writes with this command.

Correct that and add the flag, so that buffered writes are correctly
hashed when using the non-iovec based write command.

Cc: stable@vger.kernel.org
Fixes: 3a6820f2 ("io_uring: add non-vectored read/write commands")
Signed-off-by: Jens Axboe <axboe@kernel.dk>