8941 zpool add: assertion failed in get_replication() with nested interior VDEVs

illumos/illumos-gate@ac0215f4d618163d117a40fbf77a3f944852cb7b

When replacing a faulted device which was previously handled by a spare
multiple levels of nested interior VDEVs will be present in the pool
configuration: get_replication() needs to handle this situation gracefully
to let zpool add new devices to the pool

Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Andrew Stormont <andyjstormont@gmail.com>
Approved by: Dan McDonald <danmcd@joyent.com>
Author: loli10K <ezomori.nozomu@gmail.com>