CIFS: Fix NULL-pointer dereference in smb2_push_mandatory_locks

Currently when the client creates a cifsFileInfo structure for
a newly opened file, it allocates a list of byte-range locks
with a pointer to the new cfile and attaches this list to the
inode's lock list. The latter happens before initializing all
other fields, e.g. cfile->tlink. Thus a partially initialized
cifsFileInfo structure becomes available to other threads that
walk through the inode's lock list. One example of such a thread
may be an oplock break worker thread that tries to push all
cached byte-range locks. This causes NULL-pointer dereference
in smb2_push_mandatory_locks() when accessing cfile->tlink:

[598428.945633] BUG: kernel NULL pointer dereference, address: 0000000000000038
...
[598428.945749] Workqueue: cifsoplockd cifs_oplock_break [cifs]
[598428.945793] RIP: 0010:smb2_push_mandatory_locks+0xd6/0x5a0 [cifs]
...
[598428.945834] Call Trace:
[598428.945870] ? cifs_revalidate_mapping+0x45/0x90 [cifs]
[598428.945901] cifs_oplock_break+0x13d/0x450 [cifs]
[598428.945909] process_one_work+0x1db/0x380
[598428.945914] worker_thread+0x4d/0x400
[598428.945921] kthread+0x104/0x140
[598428.945925] ? process_one_work+0x380/0x380
[598428.945931] ? kthread_park+0x80/0x80
[598428.945937] ret_from_fork+0x35/0x40

Fix this by reordering initialization steps of the cifsFileInfo
structure: initialize all the fields first and then add the new
byte-range lock list to the inode's lock list.

Cc: Stable <[e-mail]>
Signed-off-by: Pavel Shilovsky <[e-mail]>
Reviewed-by: Aurelien Aptel <[e-mail]>
Signed-off-by: Steve French <[e-mail]>

fs/cifs/file.c

···
 	INIT_LIST_HEAD(&fdlocks->locks);
 	fdlocks->cfile = cfile;
 	cfile->llist = fdlocks;
-	cifs_down_write(&cinode->lock_sem);
-	list_add(&fdlocks->llist, &cinode->llist);
-	up_write(&cinode->lock_sem);
 
 	cfile->count = 1;
 	cfile->pid = current->tgid;
···
 		cifs_dbg(FYI, "Reset oplock val from read to None due to mand locks\n");
 		oplock = 0;
 	}
+
+	cifs_down_write(&cinode->lock_sem);
+	list_add(&fdlocks->llist, &cinode->llist);
+	up_write(&cinode->lock_sem);
 
 	spin_lock(&tcon->open_file_lock);
 	if (fid->pending_open->oplock != CIFS_OPLOCK_NO_CHANGE && oplock)