smtp: pre process DATA and BDAT commands

smtp: pre process DATA and BDAT commands

The input data received in DATA and BDAT command modes can be huge and
could have important data, like a legit huge email. Therefore, exempt
these from the line buffering limits which were introduced to regulate
the size of lines that we buffer at any point in time.

As a part of this patch, anything that comes under DATA or BDAT is
processed early without buffering as and when it arrives. The ways of
processing remain the same as before.