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

commit 7bd88377 upstream.

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.
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Cc: Ian Jackson <ian.jackson@eu.citrix.com>
[lizf: the previous backport of this upstream commit is buggy. fix it]
Signed-off-by: Zefan Li <lizefan@huawei.com>

fs/namei.c

@@ -579,7 +579,8 @@ static __always_inline int __vfs_follow_link(struct nameidata *nd, const char *l
 		goto fail;
 
 	if (*link == '/') {
-		set_root(nd);
+		if (!nd->root.mnt)
+			set_root(nd);
 		path_put(&nd->path);
 		nd->path = nd->root;
 		path_get(&nd->root);