dm ima: fix wrong length calculation for no_data string

All entries measured by dm ima are prefixed by a version string
(dm_version=N.N.N). When there is no data to measure, the entire buffer is
overwritten with a string containing the version string again and the
length of that string is added to the length of the version string.
The new length is now wrong because it contains the version string twice.

This caused entries like this:
dm_version=4.45.0;name=test,uuid=test;table_clear=no_data; \
\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00 \
current_device_capacity=204808;

Signed-off-by: Thore Sommer <public@thson.de>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>