SLUB: Define functions for cpu slab handling instead of using PageActive

Use inline functions to access the per cpu bit.  Intoduce the notion of
"freezing" a slab to make things more understandable.

Signed-off-by: Christoph Lameter <clameter@sgi.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>