MFV r328231: 8897 zpool online -e fails assertion when run on non-leaf vdevs

illumos/illumos-gate@9a551dd645b478816cb11251b19f5034d885bf01

https://www.illumos.org/issues/8897:

zpool online -e test mirror-1

Assertion failed: nvlist_lookup_string(tgt, "path", &pathname) == 0, file ../common/libzfs_pool.c, line 2558, function zpool_vdev_online
Abort (core dumped)

Not a big deal per se, but should be handled gracefully, same way as 'offline' and 'online' without '-e'.

Also reported as: https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=221408

Reviewed by: Toomas Soome <tsoome@me.com>
Reviewed by: Igor Kozhukhov <igor@dilos.org>
Approved by: Dan McDonald <danmcd@joyent.com>
Author: Yuri Pankov <yuri.pankov@nexenta.com>