cifs: Do not lookup hashed negative dentry in cifs_atomic_open

We do not need to lookup a hashed negative directory since we have
already revalidated it before and have found it to be fine.

This also prevents a crash in cifs_lookup() when it attempts to rehash
the already hashed negative lookup dentry.

The patch has been tested using the reproducer at
https://bugzilla.redhat.com/show_bug.cgi?id=867344#c28

Cc: <stable@kernel.org> # 3.6.x
Reported-by: Vit Zahradka <vit.zahradka@tiscali.cz>
Signed-off-by: Sachin Prabhu <sprabhu@redhat.com>

fs/cifs/dir.c

@@ -398,7 +398,16 @@ cifs_atomic_open(struct inode *inode, struct dentry *direntry,
 	 * in network traffic in the other paths.
 	 */
 	if (!(oflags & O_CREAT)) {
-		struct dentry *res = cifs_lookup(inode, direntry, 0);
+		struct dentry *res;
+
+		/*
+		 * Check for hashed negative dentry. We have already revalidated
+		 * the dentry and it is fine. No need to perform another lookup.
+		 */
+		if (!d_unhashed(direntry))
+			return -ENOENT;
+
+		res = cifs_lookup(inode, direntry, 0);
 		if (IS_ERR(res))
 			return PTR_ERR(res);