changed BCC handling for SMBwriteX to handle broken MacOSX client

changed BCC handling for SMBwriteX to handle broken MacOSX client

see bug #6610

The MacOSX SMB client sets the BCC value in SMBwriteX calls to zero
instead of the correct size. Checking against WindowsXP, I've found
that Windows uses the maximum of the computed buffer size and the
given BCC value. I've changed Samba4 to do the same to allow MacOSX to
work.

I've limited this change to non-chained packets to ensure we don't get
the possibility of exploits based on overlapping chained requests