slub: fix shadowed variable sparse warnings

Introduce 'len' at outer level:
mm/slub.c:3406:26: warning: symbol 'n' shadows an earlier one
mm/slub.c:3393:6: originally declared here

No need to declare new node:
mm/slub.c:3501:7: warning: symbol 'node' shadows an earlier one
mm/slub.c:3491:6: originally declared here

No need to declare new x:
mm/slub.c:3513:9: warning: symbol 'x' shadows an earlier one
mm/slub.c:3492:6: originally declared here

Signed-off-by: Harvey Harrison <harvey.harrison@gmail.com>
Signed-off-by: Christoph Lameter <clameter@sgi.com>