fs: Remove the buffer_head kmem_cache constructor to reduce memory usage
under slub.
When using slub, having a kmem_cache constructor forces slub to add a
free pointer to the size of the cached object, which can have a
significant impact to the number of small objects that can fit into a
slab.
As buffer_head is relatively small and we can have large numbers of
them, removing the constructor is a definite win.
On x86_64 removing the constructor gives me 39 objects/slab, 3 more than
without the patch. And on x86_32 73 objects/slab, which is 9 more.
As alloc_buffer_head() already initializes each new object there is very
little difference in actual code run.
Signed-off-by: Richard Kennedy<richard@xxxxxxxxxxxxxxx>