cifs: Set client guid on per connection basis

When mounting from a Windows 2012R2 server, we hit the following
problem:
1) Mount with any of the following versions - 2.0, 2.1 or 3.0
2) unmount
3) Attempt a mount again using a different SMB version >= 2.0.

You end up with the following failure:
Status code returned 0xc0000203 STATUS_USER_SESSION_DELETED
CIFS VFS: Send error in SessSetup = -5
CIFS VFS: cifs_mount failed w/return code = -5

I cannot reproduce this issue using a Windows 2008 R2 server.

This appears to be caused because we use the same client guid for the
connection on first mount which we then disconnect and attempt to mount
again using a different protocol version. By generating a new guid each
time a new connection is Negotiated, we avoid hitting this problem.

Signed-off-by: Sachin Prabhu <sprabhu@redhat.com>
Signed-off-by: Steve French <smfrench@gmail.com>

fs/cifs/connect.c

@@ -2144,6 +2144,9 @@ cifs_get_tcp_session(struct smb_vol *volume_info)
 	       sizeof(tcp_ses->srcaddr));
 	memcpy(&tcp_ses->dstaddr, &volume_info->dstaddr,
 		sizeof(tcp_ses->dstaddr));
+#ifdef CONFIG_CIFS_SMB2
+	get_random_bytes(tcp_ses->client_guid, SMB2_CLIENT_GUID_SIZE);
+#endif
 	/*
 	 * at this point we are the only ones with the pointer
 	 * to the struct since the kernel thread not created yet