io_uring: io_allocate_scq_urings() should return a sane state

We currently rely on the ring destroy on cleaning things up in case of
failure, but io_allocate_scq_urings() can leave things half initialized
if only parts of it fails.

Be nice and return with either everything setup in success, or return an
error with things nicely cleaned up.

Reported-by:  <syzbot+0d818c0d39399188f393@syzkaller.appspotmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>