contrib/dracut: 90: generator: only log to kmsg if debug set on cmdline

"debug" is also used by systemd itself, and there's really no reason for
the generator to write this much garbage by default

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #11898