io_uring: set proper FFS* flags on reg file update

Set FFS_* flags (e.g. FFS_ASYNC_READ) not only in initial registration
but also on registered files update. Not a bug, but may miss getting
profit out of the feature.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/df29a841a2d3d3695b509cdffce5070777d9d942.1617287883.git.asml.silence@gmail.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>