SLAB: don't allocate empty shared caches

SLAB: don't allocate empty shared caches

We can avoid allocating empty shared caches and avoid unecessary check of
cache->limit.  We save some memory.  We avoid bringing into CPU cache
unecessary cache lines.

All accesses to l3->shared are already checking NULL pointers so this patch is
safe.

Signed-off-by: Eric Dumazet <[email protected]>
Acked-by: Pekka Enberg <[email protected]>
Cc: Christoph Lameter <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>