dm: ima: avoid extra calls to strlen()

Since 'scnprintf()' returns the number of characters emitted (not
including the trailing '\0'), use that return value instead of the
subsequent calls to 'strlen()' where appropriate. Compile tested only.

Signed-off-by: Dmitry Antipov <dmantipov@yandex.ru>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>