The DMA iterator logic was mixed into verbs and umem-specific code,
forcing all users to include rdma/ib_umem.h. Move the block iterator
logic into iter.c and rdma/iter.h so that rdma/ib_umem.h and
rdma/ib_verbs.h can be separated in a follow-up patch.
Link: https://patch.msgid.link/20260213-refactor-umem-v1-1-f3be85847922@nvidia.com
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>