ktest.pl: Add MAIL_MAX_SIZE to limit the amount of log emailed

Add the ktest config option MAIL_MAX_SIZE that will limit the size of the
log file that is placed into the email on failure.

Link: https://lore.kernel.org/r/20200701231756.790637968@goodmis.org



Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>