don't bugger nd->seq on set_root_rcu() from follow_dotdot_rcu()

return the value instead, and have path_init() do the assignment.  Broken by
"vfs: Fix absolute RCU path walk failures due to uninitialized seq number",
which was Cc-stable with 2.6.38+ as destination.  This one should go where
it went.

To avoid dummy value returned in case when root is already set (it would do
no harm, actually, since the only caller that doesn't ignore the return value
is guaranteed to have nd->root *not* set, but it's more obvious that way),
lift the check into callers.  And do the same to set_root(), to keep them
in sync.

Cc: stable@vger.kernel.org # 2.6.38+
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/namei.c

@@ -643,24 +643,22 @@ static int complete_walk(struct nameidata *nd)
 
 static __always_inline void set_root(struct nameidata *nd)
 {
-	if (!nd->root.mnt)
 	get_fs_root(current->fs, &nd->root);
 }
 
 static int link_path_walk(const char *, struct nameidata *);
 
-static __always_inline void set_root_rcu(struct nameidata *nd)
+static __always_inline unsigned set_root_rcu(struct nameidata *nd)
 {
-	if (!nd->root.mnt) {
 	struct fs_struct *fs = current->fs;
-		unsigned seq;
+	unsigned seq, res;
 
 	do {
 		seq = read_seqcount_begin(&fs->seq);
 		nd->root = fs->root;
-			nd->seq = __read_seqcount_begin(&nd->root.dentry->d_seq);
+		res = __read_seqcount_begin(&nd->root.dentry->d_seq);
 	} while (read_seqcount_retry(&fs->seq, seq));
-	}
+	return res;
 }
 
 static void path_put_conditional(struct path *path, struct nameidata *nd)
@@ -860,6 +858,7 @@ follow_link(struct path *link, struct nameidata *nd, void **p)
 			return PTR_ERR(s);
 		}
 		if (*s == '/') {
+			if (!nd->root.mnt)
 				set_root(nd);
 			path_put(&nd->path);
 			nd->path = nd->root;
@@ -1143,6 +1142,7 @@ static bool __follow_mount_rcu(struct nameidata *nd, struct path *path,
 
 static int follow_dotdot_rcu(struct nameidata *nd)
 {
+	if (!nd->root.mnt)
 		set_root_rcu(nd);
 
 	while (1) {
@@ -1256,6 +1256,7 @@ static void follow_mount(struct path *path)
 
 static void follow_dotdot(struct nameidata *nd)
 {
+	if (!nd->root.mnt)
 		set_root(nd);
 
 	while(1) {
@@ -1852,7 +1853,7 @@ static int path_init(int dfd, const char *name, unsigned int flags,
 	if (*name=='/') {
 		if (flags & LOOKUP_RCU) {
 			rcu_read_lock();
-			set_root_rcu(nd);
+			nd->seq = set_root_rcu(nd);
 		} else {
 			set_root(nd);
 			path_get(&nd->root);