CIFS: SMBD: Implement function to destroy a SMB Direct connection

Add function to tear down a SMB Direct connection. This is used by upper layer
to free all SMB Direct connection and transport resources.

Signed-off-by: Long Li <longli@microsoft.com>
Signed-off-by: Steve French <smfrench@gmail.com>
Reviewed-by: Pavel Shilovsky <pshilov@microsoft.com>
Reviewed-by: Ronnie Sahlberg <lsahlber@redhat.com>