Unify /proc/slabinfo configuration

Both SLUB and SLAB really did almost exactly the same thing for
/proc/slabinfo setup, using duplicate code and per-allocator #ifdef's.

This just creates a common CONFIG_SLABINFO that is enabled by both SLUB
and SLAB, and shares all the setup code.  Maybe SLOB will want this some
day too.

Reviewed-by: Pekka Enberg <penberg@cs.helsinki.fi>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/slub_def.h

@@ -200,6 +200,4 @@ 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 */