LU-506 kernel: FC15 - support dcache scalability changes.

LU-506 kernel: FC15 - support dcache scalability changes.

* dcache supports rcu-walk, but lustre doesn't, so
revalidate/check_acl/permission fall back to ref-walk.
* dcache_lock is completely removed in latest kernel, many
fields are protected by inode->i_lock now.
* remove super hack d_rehash_cond(), and treats
DCACHE_LUSTRE_INVALID similar to DCACHE_DISCONNECTED, therefore
dentry doesn't need to be dropped and rehashed frequently.
* .lookup(LOOKUP_CREATE) calls d_add() dentry directly, and
.create only needs to d_instantiate() this dentry.
* set sb->s_d_op to ll_d_ops, and d_alloc() will initialize
dentry with it, therefore it's no need to set dentry operations
explicitely for new kernel.
* .d_delete is called before dentry refcount decrease for new
kernel.
* other cleanups.

Change-Id: Ib08bd4a45dba8d3d5b6150e9a4df95728fdf33e2
Signed-off-by: Lai Siyao <laisiyao@whamcloud.com>
Signed-off-by: yang sheng <ys@whamcloud.com>
Reviewed-on: http://review.whamcloud.com/1865
Reviewed-by: Fan Yong <yong.fan@whamcloud.com>
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>