io_uring: fix invalid error check after malloc

Now we allocate io_mapped_ubuf instead of bvec, so we clearly have to
check its address after allocation.

Fixes: 41edf1a5 ("io_uring: keep table of pointers to ubufs")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/d28eb1bc4384284f69dbce35b9f70c115ff6176f.1619392565.git.asml.silence@gmail.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>