io_uring: fix mis-seting personality's creds

After io_identity_cow() copies an work.identity it wants to copy creds
to the new just allocated id, not the old one. Otherwise it's
akin to req->work.identity->creds = req->work.identity->creds.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>