CIFS: Fix a possible memory corruption during reconnect

We can not unlock/lock cifs_tcp_ses_lock while walking through ses
and tcon lists because it can corrupt list iterator pointers and
a tcon structure can be released if we don't hold an extra reference.
Fix it by moving a reconnect process to a separate delayed work
and acquiring a reference to every tcon that needs to be reconnected.
Also do not send an echo request on newly established connections.

CC: Stable <stable@vger.kernel.org>
Signed-off-by: Pavel Shilovsky <pshilov@microsoft.com>

fs/cifs/cifsglob.h

@@ -647,6 +647,8 @@ struct TCP_Server_Info {
 	unsigned int	max_read;
 	unsigned int	max_write;
 	__u8		preauth_hash[512];
+	struct delayed_work reconnect; /* reconnect workqueue job */
+	struct mutex reconnect_mutex; /* prevent simultaneous reconnects */
 #endif /* CONFIG_CIFS_SMB2 */
 	unsigned long echo_interval;
 };
@@ -850,6 +852,7 @@ cap_unix(struct cifs_ses *ses)
 struct cifs_tcon {
 	struct list_head tcon_list;
 	int tc_count;
+	struct list_head rlist; /* reconnect list */
 	struct list_head openFileList;
 	spinlock_t open_file_lock; /* protects list above */
 	struct cifs_ses *ses;	/* pointer to session associated with */