io_uring: add generic path for rsrc update

Extract some common parts for rsrc update, will be used reg buffers
support dynamic (i.e. quiesce-lee) managing.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/b49c3ff6b9ff0e530295767604fe4de64d349e04.1619356238.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>