slub: provide /proc/slabinfo

This adds a read-only /proc/slabinfo file on SLUB, that makes slabtop work.

[ mingo@elte.hu: build fix. ]

Cc: Andi Kleen <andi@firstfloor.org>
Cc: Christoph Lameter <clameter@sgi.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Pekka Enberg <penberg@cs.helsinki.fi>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/slub_def.h

@@ -200,4 +200,6 @@ static __always_inline void *kmalloc_node(size_t size, gfp_t flags, int node)
 }
 #endif
 
+extern const struct seq_operations slabinfo_op;
+
 #endif /* _LINUX_SLUB_DEF_H */