io_uring: free iovec if file assignment fails

We just return failure in this case, but we need to release the iovec
first. If we're doing IO with more than FAST_IOV segments, then the
iovec is allocated and must be freed.

Reported-by:  <syzbot+96b43810dfe9c3bb95ed@syzkaller.appspotmail.com>
Fixes: 584b0180 ("io_uring: move read/write file prep state into actual opcode handler")
Signed-off-by: Jens Axboe <axboe@kernel.dk>