LinuxKPI: skbuff: rewrite data allocation to use __kmalloc by default

Put tunable/contigmalloc code under SKB_DMA32_MALLOC and remove the
LP64 checks for simplicity. #define SKB_DMA32_MALLOC to be set
so that the code stays enabled as busdma still has trouble bouncing
contiguous memory with nseg=1 if it is multiple pages.

Switch the other code to use __kmalloc() which now automatically
provides physically contiguous memory and deals with freeing as well.
This code should in theroy be enough now.

If people use iwlwifi(4) pre-22000 (9xxx 8xxx ...) chipsets on Laptops
with more than 64GB of memory they may see bounce errors as well now.

Sposnored by: The FreeBSD Foundation
MFC after: 3 days