slab: clean up the code comment in slab kmem_cache struct

slab: clean up the code comment in slab kmem_cache struct

In commit 3b0efdfa1e7 ("mm, sl[aou]b: Extract common fields from struct
kmem_cache") the variable 'obj_size' was moved above, however the
related code comment is not updated accordingly.  Do it here.

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Baoquan He <[email protected]>
Reviewed-by: Andrew Morton <[email protected]>
Acked-by: Christoph Lameter <[email protected]>
Cc: Pekka Enberg <[email protected]>
Cc: David Rientjes <[email protected]>
Cc: Joonsoo Kim <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>