LU-645 Avoid unnecessary dentry rehashing

LU-645 Avoid unnecessary dentry rehashing

In patchless case the sequence __d_drop() then d_rehash_cond() creates
race window if dentry is already hashed, in this case rehashing should
be avoided.

Change-Id: Ib891e7d2e5ae7efe19ef2ac0abb111e9f9681b7b
Signed-off-by: Bobi Jam <bobijam@whamcloud.com>
Reviewed-on: http://review.whamcloud.com/1435
Tested-by: Hudson
Reviewed-by: Fan Yong <yong.fan@whamcloud.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Johann Lombardi <johann@whamcloud.com>