cifs: fix double free race when mount fails in cifs_get_root()

stable inclusion
from linux-4.19.233
commit 2fe0e281f7ad0a62259649764228227dd6b2561d
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I5646A
CVE: NA

--------------------------------

[ Upstream commit 3d6cc9898efdfb062efb74dc18cfc700e082f5d5 ]

When cifs_get_root() fails during cifs_smb3_do_mount() we call
deactivate_locked_super() which eventually will call delayed_free() which
will free the context.
In this situation we should not proceed to enter the out: section in
cifs_smb3_do_mount() and free the same resources a second time.

[Thu Feb 10 12:59:06 2022] BUG: KASAN: use-after-free in rcu_cblist_dequeue+0x32/0x60
[Thu Feb 10 12:59:06 2022] Read of size 8 at addr ffff888364f4d110 by task swapper/1/0

[Thu Feb 10 12:59:06 2022] CPU: 1 PID: 0 Comm: swapper/1 Tainted: G           OE     5.17.0-rc3+ #4
[Thu Feb 10 12:59:06 2022] Hardware name: Microsoft Corporation Virtual Machine/Virtual Machine, BIOS Hyper-V UEFI Release ...

fs/cifs/cifsfs.c

@@ -779,6 +779,7 @@ cifs_smb3_do_mount(struct file_system_type *fs_type,
 
 out_super:
 	deactivate_locked_super(sb);
+	return root;
 out:
 	cifs_cleanup_volume_info(volume_info);
 	return root;