zpool iostat: update pool counter when skipping boot row

When skipping the boot row (with -y), the early loop meant we weren't
updating the "last_npools" count. That means the count never advanced
past zero, so cb_iteration was always reset to 0, leading to it being
"stuck" on the boot line, printing the header and nothing else forever.

Updating the pool counter on every loop sorts that out: it advances,
cb_iteration moves properly, and normal rows are printed.

Sponsored-by: Klara, Inc.
Sponsored-by: Wasabi Technology, Inc.
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rob Norris <rob.norris@klarasystems.com>
Closes #17807

cmd/zpool/zpool_main.c

@@ -6677,6 +6677,7 @@ zpool_do_iostat(int argc, char **argv)
 			if (skip) {
 				(void) fflush(stdout);
 				(void) fsleep(interval);
+				last_npools = npools;
 				continue;
 			}