linuxkpi: Fix return value of dma_map_sgtable

[ashafer_badland.io]

Summary

dma_map_sgtable internally uses the dma_map_sg_attrs helper. The problem is
that dma_map_sg_attrs returns the number of entries mapped, whereas
dma_map_sgtable returns nonzero on failure. This leads to dma_map_sgtable
returning non-zero-but-positive values which tricks other areas of the stack
into thinking nents is a valid pointer.

This checks if nents is valid and returns zero if so, updating the nents field
in sgt. This fixes PRIME render offload with nvidia-drm

Test Plan

PRIME rendering offload with i915kms and nvidia-drm

[ashafer_badland.io]

Here's the github PR handling this for older versions before we moved this function into base: https://github.com/freebsd/drm-kmod/pull/218

[bz]

Functionally looks fine; style may be improved (or not).

sys/compat/linuxkpi/common/include/linux/dma-mapping.h

365

Normally I'd say separate declaration:

int nents;

nents = ...

368

() around return values here and below.

369

No need for the else clause so all the {} {} can go for both.

[ashafer_badland.io]

Closing this since it was already submitted in 4085bde9fa2e