io_uring/rsrc: simplify the bvec iter count calculation

As we don't use iov_iter_advance() but our own logic in io_import_fixed(),
we can remove the logic that over-sets the iter's count to len + offset
then adjusts it later to len. This helps to make the code cleaner.

Signed-off-by: Bui Quang Minh <minhquangbui99@gmail.com>
Link: https://lore.kernel.org/r/20250103150412.12549-1-minhquangbui99@gmail.com


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