dma-direct: Fix missing sg_dma_len assignment in P2PDMA bus mappings

Prior to commit a25e7962db ("PCI/P2PDMA: Refactor the p2pdma mapping
helpers"), P2P segments were mapped using the pci_p2pdma_map_segment()
helper. This helper was responsible for populating sg->dma_address,
marking the bus address, and also setting sg_dma_len(sg).

The refactor[1] removed this helper and moved the mapping logic directly
into the callers. While iommu_dma_map_sg() was correctly updated to set
the length in the new flow, it was missed in dma_direct_map_sg().

Thus, in dma_direct_map_sg(), the PCI_P2PDMA_MAP_BUS_ADDR case sets the
dma_address and marks the segment, but immediately executes 'continue',
which causes the loop to skip the standard assignment logic at the end:

    sg_dma_len(sg) = sg->length;

As a result, when CONFIG_NEED_SG_DMA_LENGTH is enabled, the dma_length
field remains uninitialized (zero) for P2P bus address mappings. This
breaks upper-layer drivers (for e.g. RDMA/IB) that rely on sg_dma_len()
to determine the transfer size.

Fix this by explicitly setting the DMA length in the
PCI_P2PDMA_MAP_BUS_ADDR case before continuing to the next scatterlist
entry.

Fixes: a25e7962db ("PCI/P2PDMA: Refactor the p2pdma mapping helpers")
Reported-by: Jacob Moroni <[email protected]>
Signed-off-by: Pranjal Shrivastava <[email protected]>

[1]
https://lore.kernel.org/all/ac14a0e94355bf898de65d023ccf8a2ad22a3ece.1746424934.git.leon@kernel.org/

Reviewed-by: Logan Gunthorpe <[email protected]>
Reviewed-by: Leon Romanovsky <[email protected]>
Reviewed-by: Shivaji Kant <[email protected]>
Signed-off-by: Marek Szyprowski <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

kernel/dma/direct.c

@@ -481,6 +481,7 @@ int dma_direct_map_sg(struct device *dev, struct scatterlist *sgl, int nents,
 		case PCI_P2PDMA_MAP_BUS_ADDR:
 			sg->dma_address = pci_p2pdma_bus_addr_map(&p2pdma_state,
 					sg_phys(sg));
+			sg_dma_len(sg) = sg->length;
 			sg_dma_mark_bus_address(sg);
 			continue;
 		default: