block: add for_each_mp_bvec()

Add helper of for_each_mp_bvec() for io_uring to import fixed kernel
buffer.

Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20250325135155.935398-3-ming.lei@redhat.com


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