fs: buffer_head: remove 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@rsk.demon.co.uk>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Jens Axboe <jens.axboe@oracle.com>
Acked-by: Nick Piggin <npiggin@suse.de>
Cc: "Theodore Ts'o" <tytso@mit.edu>
Reviewed-by: Rik van Riel <riel@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

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