pool_iter_refresh: don't refresh pools twice

In "all pools" mode, pool_iter_refresh() will call zpool_iter(), which
will call zpool_refresh_stats() before calling add_pool(). If we already
have the pool, this is a different handle, so we just release it and
return. Back in pool_iter_refresh(), we then call zpool_stats_refresh()
again for our handle on the same pool.

All together, this means we're doing two ZFS_IOC_POOL_STATS calls into
the kernel for every pool in the system. This isn't wrong, but it does
double the pressure on global locks.

Instead, we add a new function zpool_refresh_stats_from_handle() that
simply copies the pool config and state from one handle to another, and
use it to update our handle before we release it in add_pool(), so we
only have one call per pool per interval.

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_iter.c

@@ -103,6 +103,8 @@ add_pool(zpool_handle_t *zhp, zpool_list_t *zlp)
 		new->zn_last_refresh = zlp->zl_last_refresh;
 		uu_avl_insert(zlp->zl_avl, new, idx);
 	} else {
+		zpool_refresh_stats_from_handle(node->zn_handle, zhp);
+		node->zn_last_refresh = zlp->zl_last_refresh;
 		zpool_close(zhp);
 		free(new);
 		return (-1);