RDMA/core: Add umem "is_contiguous" and "start_dma_addr" helpers

RDMA/core: Add umem "is_contiguous" and "start_dma_addr" helpers

In some cases drivers may need to check if a given umem is contiguous.
Add a helper function in core code so that drivers don't need to deal
with umem or scatter-gather lists structure.
Additionally add a helper for getting umem's start DMA address and use
it in other helper functions that open code it.

Signed-off-by: Michael Margolin <mrgolin@amazon.com>
Link: https://patch.msgid.link/20250708202308.24783-3-mrgolin@amazon.com
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>