io_uring/rsrc: check size when importing reg buffer

We're relying on callers to verify the IO size, do it inside of
io_import_fixed() instead. It's safer, easier to deal with, and more
consistent as now it's done close to the iter init site.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/f9c2c75ec4d356a0c61289073f68d98e8a9db190.1743446271.git.asml.silence@gmail.com


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