RDMA/core: Introduce generic buffer descriptor infrastructure for umem

RDMA/core: Introduce generic buffer descriptor infrastructure for umem

Introduce a per-attribute UVERBS_ATTR_UMEM model so each uverbs
command's umem set is explicit in its UAPI definition. Add
driver-facing wrapper helpers that pin a umem on demand from an
attribute or a VA addr; the driver owns the returned umem and
releases it from its destroy/error paths.

Link: https://patch.msgid.link/r/20260529134312.2836341-4-jiri@resnulli.us
Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>