linuxkpi: Fix set_memory_*

[tijl]

Summary

set_memory_* is currently implemented using PHYS_TO_DMAP but not all
architectures have a DMAP. Looking at how this function is used the
given address isn't physical, but virtual so the PHYS_TO_DMAP call
can simply be removed.

Also cast numpages before shifting it to avoid overflow.

[tijl]

Example use:
https://github.com/freebsd/drm-kmod/blob/1f0859f54b6f4bcce3862f30c9c1b5cfd08710db/drivers/gpu/drm/radeon/radeon_gart.c#L83
set_memory_uc called on ptr returned by linuxkpi dma_alloc_coherent, which returns a virtual address.