io_uring: abort file assignment prior to assigning creds

We need to either restore creds properly if we fail on the file
assignment, or just do the file assignment first instead. Let's do
the latter as it's simpler, should make no difference here for
file assignment.

Link: https://lore.kernel.org/lkml/000000000000a7edb305dca75a50@google.com/


Reported-by:  <syzbot+60c52ca98513a8760a91@syzkaller.appspotmail.com>
Fixes: 6bf9c47a ("io_uring: defer file assignment")
Signed-off-by: Jens Axboe <axboe@kernel.dk>