io_uring/kbuf: Fix an NULL vs IS_ERR() bug in io_alloc_pbuf_ring()

The io_mem_alloc() function returns error pointers, not NULL.  Update
the check accordingly.

Fixes: b10b73c ("io_uring/kbuf: recycle freed mapped buffer ring entries")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Link: https://lore.kernel.org/r/5ed268d3-a997-4f64-bd71-47faa92101ab@moroto.mountain
Signed-off-by: Jens Axboe <axboe@kernel.dk>

io_uring/kbuf.c

@@ -636,8 +636,8 @@ static int io_alloc_pbuf_ring(struct io_ring_ctx *ctx,
 	ibf = io_lookup_buf_free_entry(ctx, ring_size);
 	if (!ibf) {
 		ptr = io_mem_alloc(ring_size);
-		if (!ptr)
-			return -ENOMEM;
+		if (IS_ERR(ptr))
+			return PTR_ERR(ptr);
 
 		/* Allocate and store deferred free entry */
 		ibf = kmalloc(sizeof(*ibf), GFP_KERNEL_ACCOUNT);