Optimise unmapped LinuxKPI page allocations.

When allocating unmapped pages, take advantage of the direct map on
AMD64 to get the virtual address corresponding to a page. Else all
pages allocated must be mapped because sometimes the virtual address
of a page is requested.

Move all page allocation and deallocation code into an own C-file.

Add support for GFP_DMA32, GFP_KERNEL, GFP_ATOMIC and __GFP_ZERO
allocation flags.

Make a clear separation between mapped and unmapped allocations.

Obtained from: kmacy @
MFC after: 1 week
Sponsored by: Mellanox Technologies