disabled resilver_defer feature leads to looping resilvers
When a disk is replaced with another on a pool with the resilver_defer
feature present, but not enabled the resilver activity restarts during
each spa_sync. This patch checks to make sure that the resilver_defer
feature is first enabled before requesting a deferred resilver.
This was originally fixed in illumos-joyent as OS-7982 (https://smartos.org/bugview/OS-7982).
This has been added to ZoL already as well. We should pull the fix into illumos-gate.