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/zpool/zpool_iter.c

@@ -379,8 +379,8 @@ process_unique_cmd_columns(vdev_cmd_data_list_t *vcdl)
 static int
 vdev_process_cmd_output(vdev_cmd_data_t *data, char *line)
 {
-	char *col = NULL;
-	char *val = line;
+	char *col;
+	char *val;
 	char *equals;
 	char **tmp;
 
@@ -397,6 +397,7 @@ vdev_process_cmd_output(vdev_cmd_data_t *data, char *line)
 		col = line;
 		val = equals + 1;
 	} else {
+		col = NULL;
 		val = line;
 	}