RDMA/core: Add UVERBS_ATTR_RAW_FD

This uses the same passing protocol as UVERBS_ATTR_FD (eg len = 0 data_s64
= fd), except that the FD is not required to be a uverbs object and the
core code does not covert the FD to an object handle automatically.

Access to the int fd is provided by uverbs_get_raw_fd().

Link: https://lore.kernel.org/r/2-v1-bd147097458e+ede-umem_dmabuf_jgg@nvidia.com


Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>