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/smb2pdu.h

@@ -183,8 +183,6 @@ struct smb2_symlink_err_rsp {
 
 #define SMB2_CLIENT_GUID_SIZE 16
 
-extern __u8 cifs_client_guid[SMB2_CLIENT_GUID_SIZE];
-
 struct smb2_negotiate_req {
 	struct smb2_hdr hdr;
 	__le16 StructureSize; /* Must be 36 */