prepend_path() needs to reinitialize dentry/vfsmount/mnt on restarts

... and equivalent is needed in 3.12; it's broken there as well
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/dcache.c

@@ -2912,9 +2912,9 @@ static int prepend_path(const struct path *path,
 			const struct path *root,
 			char **buffer, int *buflen)
 {
-	struct dentry *dentry = path->dentry;
-	struct vfsmount *vfsmnt = path->mnt;
-	struct mount *mnt = real_mount(vfsmnt);
+	struct dentry *dentry;
+	struct vfsmount *vfsmnt;
+	struct mount *mnt;
 	int error = 0;
 	unsigned seq, m_seq = 0;
 	char *bptr;
@@ -2929,6 +2929,9 @@ static int prepend_path(const struct path *path,
 	bptr = *buffer;
 	blen = *buflen;
 	error = 0;
+	dentry = path->dentry;
+	vfsmnt = path->mnt;
+	mnt = real_mount(vfsmnt);
 	read_seqbegin_or_lock(&rename_lock, &seq);
 	while (dentry != root->dentry || vfsmnt != root->mnt) {
 		struct dentry * parent;