aio: checking for NULL instead of IS_ERR

alloc_anon_inode() returns an ERR_PTR(), it doesn't return NULL.

Fixes: 71ad7490

 ('rework aio migrate pages to use aio fs')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>