slab.h contains a series of definitions that are simply aliases
for gfp_XX. This duplication makes it difficult to search for uses
of f.e. GFP_ATOMIC. Strange mixtures of uses of __GFP_xx with SLAB_xx
exist in some pieces of code.

This patchset removes all the aliases from slab.h and also removes all
occurrences.

I have tried to order these by size so that it is possible to just take the
first few of these patches should they be considered to be too complex.