io-wq: fix double put of 'wq' in error path

We are already freeing the wq struct in both spots, so don't put it and
get it freed twice.

Reported-by:  <syzbot+7bf785eedca35ca05501@syzkaller.appspotmail.com>
Fixes: 4fb6ac32 ("io-wq: improve manager/worker handling over exec")
Signed-off-by: Jens Axboe <axboe@kernel.dk>