staging/lustre/llite: lock i_lock before __d_drop()

There has been several Lustre Client crashes reported by sites
running with Lustre versions 2.1/2.5, all showing the same
dentry->d_hash->next corrupted pointer cause.

This patch fixes a regression that has been introduced since a
long time by commit :
(LU-506 kernel: FC15 - support dcache scalability changes.)

where i_lock protection usage has been removed and
that is likely to cause racy condition during dentry [un]hashing
and to be the root cause of these crashes.
Signed-off-by: Bruno Faccini <bruno.faccini@intel.com>
Reviewed-on: http://review.whamcloud.com/19287
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-7973Reviewed-by: Lai Siyao <lai.siyao@intel.com>
Reviewed-by: Yang Sheng <yang.sheng@intel.com>
Signed-off-by: Oleg Drokin <green@linuxhacker.ru>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/lustre/lustre/llite/file.c

@@ -2963,8 +2963,11 @@ static int __ll_inode_revalidate(struct dentry *dentry, __u64 ibits)
 		 * here to preserve get_cwd functionality on 2.6.
 		 * Bug 10503
 		 */
-		if (!d_inode(dentry)->i_nlink)
+		if (!d_inode(dentry)->i_nlink) {
+			spin_lock(&inode->i_lock);
 			d_lustre_invalidate(dentry, 0);
+			spin_unlock(&inode->i_lock);
+		}
 
 		ll_lookup_finish_locks(&oit, inode);
 	} else if (!ll_have_md_lock(d_inode(dentry), &ibits, LCK_MINMODE)) {