s4:torture/smb2: replace torture:cn_max_buffer_size option with the negotiated max...

s4:torture/smb2: replace torture:cn_max_buffer_size option with the negotiated max trans size

Both:

 smbtorture //w2012r2-188/torture -Ua%b smb2.notify.valid-req --smb-ports=139

and:

 smbtorture //w2012r2-188/torture -Ua%b smb2.notify.valid-req --smb-ports=445

work against Windows 2008R2, 2012R2 and 2016 now and it reflects what
we have implemented in Samba.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=13864

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>