cifs: Add helper function to check smb1+ server

SMB1 server's header_preamble_size is not 0, add use is_smb1 function
to simplify the code, no actual functional changes.

Reviewed-by: Paulo Alcantara (SUSE) <pc@cjr.nz>
Signed-off-by: Zhang Xiaoxu <zhangxiaoxu5@huawei.com>
Signed-off-by: Steve French <stfrench@microsoft.com>