infiniband: Use user_backed_iter() to see if iterator is UBUF/IOVEC

Use user_backed_iter() to see if iterator is UBUF/IOVEC rather than poking
inside the iterator.

Signed-off-by: David Howells <dhowells@redhat.com>
Link: https://lore.kernel.org/r/20230925120309.1731676-5-dhowells@redhat.com


cc: Dennis Dalessandro <dennis.dalessandro@cornelisnetworks.com>
cc: Jason Gunthorpe <jgg@ziepe.ca>
cc: Leon Romanovsky <leon@kernel.org>
cc: linux-rdma@vger.kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>