ktest.pl: Fix the logic for truncating the size of the log file for email

The logic for truncating the log file for emailing based on the
MAIL_MAX_SIZE option is confusing and incorrect. Simplify it and have the
tail of the log file truncated to the max size specified in the config.

Cc: stable@vger.kernel.org
Fixes: 855d8abd ("ktest.pl: Change the logic to control the size of the log file emailed")
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>