Do not send SMB2 signatures for SMB3 frames

Restructure code to make SMB2 vs. SMB3 signing a protocol
specific op.  SMB3 signing (AES_CMAC) is not enabled yet,
but this restructuring at least makes sure we don't send
an smb2 signature on an smb3 signed connection. A followon
patch will add AES_CMAC and enable smb3 signing.

Signed-off-by: Steve French <smfrench@gmail.com>
Acked-by: Jeff Layton <jlayton@samba.org>