LU-32 osd: keep root node BH ref of IAM container

LU-32 osd: keep root node BH ref of IAM container

IAM in ldiskfs-osd will always consume some slots in bh_lru (see:
fs/buffer.c), if we keep buffer_head reference on root node,
we can save one slot in bh_lru and could be somehow helpful for
overall performance, I did some tests, LRU hits rate increased
5%-10% while creating files if we always keep this reference.

Signed-off-by: Liang Zhen <liang@whamcloud.com>
Change-Id: I954f26932462169c9bfc6bbfe1a57b5348624179
Reviewed-on: http://review.whamcloud.com/1826
Tested-by: Hudson
Reviewed-by: Bobi Jam <bobijam@whamcloud.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Fan Yong <yong.fan@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Reviewed-on: http://review.whamcloud.com/4514
Reviewed-by: Bob Glossman <bob.glossman@intel.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>