io_uring: return error pointer from io_mem_alloc()

Commit e27cef86 upstream.

In preparation for having more than one time of ring allocator, make the
existing one return valid/error-pointer rather than just NULL.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>