io_uring: Fix comment of io_get_sqe

The sqe_ptr argument has been gone since 709b302f (io_uring:
simplify io_get_sqring, 2020-04-08), made the return value of the
function. Update the comment accordingly.
Signed-off-by: Fam Zheng <fam.zheng@bytedance.com>
Reviewed-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/20210604164256.12242-1-fam.zheng@bytedance.comSigned-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -6673,7 +6673,7 @@ static void io_commit_sqring(struct io_ring_ctx *ctx)
 }
 
 /*
- * Fetch an sqe, if one is available. Note that sqe_ptr will point to memory
+ * Fetch an sqe, if one is available. Note this returns a pointer to memory
  * that is mapped by userspace. This means that care needs to be taken to
  * ensure that reads are stable, as we cannot rely on userspace always
  * being a good citizen. If members of the sqe are validated and then later