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.

(cherry picked from commit cf749fd450ca41dc001c2a1c10d8d17500dcedce)