cifs: add a new SMB2_close_flags function

And make SMB2_close just a wrapper for SMB2_close_flags.
We need this as we will start to send SMB2_CLOSE pdus using special
flags.

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