dnf: only write the log lock to root for native dnf

From commit 742a1b71, log_lock.pid is writen to root, but following
file is not changed, which will make it never deleted, and an unexpected
file exist in root dir after boot target.

$ tail  -n 1 etc/tmpfiles.d/dnf.conf
r /var/log/log_lock.pid

Besides, root dir may be read-only, so it is better still
keep the log_lock.pid under /var/log, only write the log lock to root
for native dnf for fixing issue mentioned in 742a1b71

Signed-off-by: Changqing Li <changqing.li@windriver.com>
Signed-off-by: Steve Sakoman <steve@sakoman.com>