cifs: update smb2_calc_size to use smb2_sync_hdr instead of smb2_hdr

smb2_hdr is just a wrapper around smb2_sync_hdr at this stage
and smb2_hdr is going away.

Signed-off-by: Ronnie Sahlberg <lsahlber@redhat.com>
Signed-off-by: Steve French <stfrench@microsoft.com>