Small fixes for the IOMMU code:

1.) Handle maximum segment sizes which are smaller than the IOMMU page

size by splitting up pages across multiple segments if needed; this case
was previously unimplemented, and would cause panics.

2.) KASSERT that the physical address is in range; remove a KASSERT that

has become pointless.

3.) Add a comment describing what remains to be fixed in the IOMMU code;

I plan to address these issues soon.

Desired by: dwhite (1)