11541 allocation_classes feature must be enabled to add log device
illumos/illumos-gate@c1064fd7ce62fe763a4475e9988ffea3b22137de
https://github.com/illumos/illumos-gate/commit/c1064fd7ce62fe763a4475e9988ffea3b22137de
https://www.illumos.org/issues/11541
After the allocation_classes feature was integrated, one can no longer add a log device to a pool unless that feature is enabled. There is an explicit check for this, but it is unnecessary in the case of log devices, so we should handle this better instead of forcing the feature to be enabled.
Author: Jerry Jelinek <jerry.jelinek@joyent.com>