ORNL-7 replace statahead hacking with its own cache

ORNL-7 replace statahead hacking with its own cache

Original statahead hacks dcache, it inserts dentry into dcache and
associates with inode without holding parent's lock, which breaks
VFS layer synchronization mechanism.

The new statahead does not build dentry, it just pre-fetches inode's
attributes and related ldlm locks, caches them in a small statahead
cache against parent's inode. The statahead sponsor can search such
small cache similar as searching dcache to look for what it wants.
If cache hit, then builds dentry, associates with inode, and inserts
into dcache by the statahead sponsor itself.

Signed-off-by: Fan Yong <yong.fan@whamcloud.com>
Change-Id: I6948f8a438a938c51563468d775e676e4185e580
Reviewed-on: http://review.whamcloud.com/1208
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Lai Siyao <laisiyao@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>