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

include/libzfs.h

@@ -479,6 +479,8 @@ _LIBZFS_H zpool_status_t zpool_import_status(nvlist_t *, const char **,
 _LIBZFS_H nvlist_t *zpool_get_config(zpool_handle_t *, nvlist_t **);
 _LIBZFS_H nvlist_t *zpool_get_features(zpool_handle_t *);
 _LIBZFS_H int zpool_refresh_stats(zpool_handle_t *, boolean_t *);
+_LIBZFS_H void zpool_refresh_stats_from_handle(zpool_handle_t *,
+    zpool_handle_t *);
 _LIBZFS_H int zpool_get_errlog(zpool_handle_t *, nvlist_t **);
 _LIBZFS_H void zpool_add_propname(zpool_handle_t *, const char *);