Patch from Manfred Spraul <manfred@colorfullife.com>

Effectively adds another 24 megabytes of ia32 KVA.

kmap uses a 4 MB (2 MB if PAE is enabled) area for the persistant mappings.
But PKMAP_BASE is 4GB-32MB, i.e.  around 28 MB are reserved for kmap (4 MB
for fixmappings).

The attached patch shrink that to 4 MB (+4MB for fixmappings) and adds a test
to check for an overlap between fixmap and kmap areas.