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

Fixes: b10b73c1 ("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.mountainSigned-off-by: Jens Axboe <axboe@kernel.dk>