Revert "io_uring/rsrc: simplify the bvec iter count calculation"

Revert "io_uring/rsrc: simplify the bvec iter count calculation"

This reverts commit 2a51c327d4a4a2eb62d67f4ea13a17efd0f25c5c.

The kernel registered bvecs do use the iov_iter_advance() API, so we
can't rely on this simplification anymore.

Fixes: 27cb27b6d5ea40 ("io_uring: add support for kernel registered bvecs")
Reported-by: Caleb Sander Mateos <csander@purestorage.com>
Signed-off-by: Keith Busch <kbusch@kernel.org>
Reviewed-by: Caleb Sander Mateos <csander@purestorage.com>
Link: https://lore.kernel.org/r/20250310184825.569371-1-kbusch@meta.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>