RDMA/mlx5: Implement DMABUF export ops

RDMA/mlx5: Implement DMABUF export ops

Enable p2pdma on the mlx5 PCI device to allow DMABUF-based peer-to-peer
DMA mappings.

Add implementation of the mmap_get_pfns and pgoff_to_mmap_entry device
operations required for DMABUF support in the mlx5 RDMA driver.

The pgoff_to_mmap_entry operation converts a page offset to the
corresponding rdma_user_mmap_entry by extracting the command and index
from the offset and looking it up in the ucontext's mmap_xa.

The mmap_get_pfns operation retrieves the physical address and length
from the mmap entry and obtains the p2pdma provider for the underlying
PCI device, which is needed for peer-to-peer DMA operations with
DMABUFs.

Signed-off-by: Yishai Hadas <yishaih@nvidia.com>
Signed-off-by: Edward Srouji <edwards@nvidia.com>
Link: https://patch.msgid.link/20260201-dmabuf-export-v3-3-da238b614fe3@nvidia.com
Signed-off-by: Leon Romanovsky <leon@kernel.org>