RDMA/umem: Block plain userspace memory registration under CoCo bounce

When a device requires DMA bounce buffering inside a Confidential
Computing guest, __ib_umem_get_va() cannot work. The DMA mapping layer
redirects all mappings through swiotlb bounce buffers, so the device
receives DMA addresses pointing to bounce buffer memory rather than the
user's pages. Since RDMA devices access registered memory directly without
CPU involvement, there is no opportunity for swiotlb to synchronize
between the bounce buffer and the original pages.

The registration would already fail later on, since the umem mapping is
requested with DMA_ATTR_REQUIRE_COHERENT and gets rejected under
is_swiotlb_force_bounce() with -EIO. Fail early with -EOPNOTSUPP instead,
so the user gets a specific error code to react to.

Link: https://patch.msgid.link/r/20260517141311.2409230-3-jiri@resnulli.us
Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>

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