audit: no leading space in audit_log_d_path prefix

audit_log_d_path() injects an additional space before the prefix,
which serves no purpose and doesn't mix well with other audit_log*()
functions that do not sneak extra characters into the log.

Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Eric Paris <eparis@redhat.com>