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: NEric Dumazet <dada1@cosmosbay.com>
Acked-by: NPekka Enberg <penberg@cs.helsinki.fi>
Cc: Christoph Lameter <clameter@engr.sgi.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>