smb/server: fix minimum SMB2 PDU size

smb/server: fix minimum SMB2 PDU size

The minimum SMB2 PDU size should be updated to the size of
`struct smb2_pdu` (that is, the size of `struct smb2_hdr` + 2).

Suggested-by: David Howells <dhowells@redhat.com>
Suggested-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: ChenXiaoSong <chenxiaosong@kylinos.cn>
Reviewed-by: David Howells <dhowells@redhat.com>
Acked-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>