everywhere: misc unnecessary var init/update

These are all cases where we initialise or update a variable, and then
never use it. None of them particularly matter, as the compiler should
optimise them all away during dead store elimination, but some static
analysers complain about them and they are extra work for casual readers
to follow, so worth removing.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Rob Norris <robn@despairlabs.com>
Sponsored-by: https://despairlabs.com/sponsor/
Closes #17551

cmd/zed/zed_event.c

@@ -110,7 +110,7 @@ zed_event_fini(struct zed_conf *zcp)
 static void
 _bump_event_queue_length(void)
 {
-	int zzlm = -1, wr;
+	int zzlm, wr;
 	char qlen_buf[12] = {0}; /* parameter is int => max "-2147483647\n" */
 	long int qlen, orig_qlen;