aio: fix failure to put the file pointer

If the ioprio capability check fails, we return without putting
the file pointer.

Fixes: d9a08a9e

 ("fs: Add aio iopriority support")
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>