ceph analog of cifs build_path_from_dentry() race fix

... unfortunately, cifs bug got copied.  Fix is essentially the same.
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/ceph/mds_client.c

@@ -1438,12 +1438,15 @@ char *ceph_mdsc_build_path(struct dentry *dentry, int *plen, u64 *base,
 	struct dentry *temp;
 	char *path;
 	int len, pos;
+	unsigned seq;
 
 	if (dentry == NULL)
 		return ERR_PTR(-EINVAL);
 
 retry:
 	len = 0;
+	seq = read_seqbegin(&rename_lock);
+	rcu_read_lock();
 	for (temp = dentry; !IS_ROOT(temp);) {
 		struct inode *inode = temp->d_inode;
 		if (inode && ceph_snap(inode) == CEPH_SNAPDIR)
@@ -1455,10 +1458,12 @@ char *ceph_mdsc_build_path(struct dentry *dentry, int *plen, u64 *base,
 			len += 1 + temp->d_name.len;
 		temp = temp->d_parent;
 		if (temp == NULL) {
+			rcu_read_unlock();
 			pr_err("build_path corrupt dentry %p\n", dentry);
 			return ERR_PTR(-EINVAL);
 		}
 	}
+	rcu_read_unlock();
 	if (len)
 		len--;  /* no leading '/' */
 
@@ -1467,9 +1472,12 @@ char *ceph_mdsc_build_path(struct dentry *dentry, int *plen, u64 *base,
 		return ERR_PTR(-ENOMEM);
 	pos = len;
 	path[pos] = 0;	/* trailing null */
+	rcu_read_lock();
 	for (temp = dentry; !IS_ROOT(temp) && pos != 0; ) {
-		struct inode *inode = temp->d_inode;
+		struct inode *inode;
 
+		spin_lock(&temp->d_lock);
+		inode = temp->d_inode;
 		if (inode && ceph_snap(inode) == CEPH_SNAPDIR) {
 			dout("build_path path+%d: %p SNAPDIR\n",
 			     pos, temp);
@@ -1478,21 +1486,26 @@ char *ceph_mdsc_build_path(struct dentry *dentry, int *plen, u64 *base,
 			break;
 		} else {
 			pos -= temp->d_name.len;
-			if (pos < 0)
+			if (pos < 0) {
+				spin_unlock(&temp->d_lock);
 				break;
+			}
 			strncpy(path + pos, temp->d_name.name,
 				temp->d_name.len);
 		}
+		spin_unlock(&temp->d_lock);
 		if (pos)
 			path[--pos] = '/';
 		temp = temp->d_parent;
 		if (temp == NULL) {
+			rcu_read_unlock();
 			pr_err("build_path corrupt dentry\n");
 			kfree(path);
 			return ERR_PTR(-EINVAL);
 		}
 	}
-	if (pos != 0) {
+	rcu_read_unlock();
+	if (pos != 0 || read_seqretry(&rename_lock, seq)) {
 		pr_err("build_path did not end path lookup where "
 		       "expected, namelen is %d, pos is %d\n", len, pos);
 		/* presumably this is only possible if racing with a