SSV-22933: Do not include special mirror vdev size in ZPOOL_GET_SIZE_STATS ioctl response.

[datacore-rm]

Jira: SSV-22933 : zpool allocations exceed 85% when CO is configured with special mirror vdev

Issue: Special mirror vdev size/alloc is included in the zpool size reported to the pool driver in the ZPOOL_GET_SIZE_STATS ioctl response. This results in pool driver exceeding the 85% zpool allocation thresh hold.

Fix: Since special vdev is used only to store metadata, it's size should not be included.

Tests by Dev: -
Tested on both RAID0 and RAID5 with/without special mirror vdev to validate that correct zpool sizes and alloc details are reported in the ioctl response. Zpool allocations did not exceed thresh hold of ~85%.

Tests to be done by QA:

1. Add the new low space test cases with special mirror vdev in the ILDC DCAF test suite.
2. Manual testing.

[datacore-ramya]

metaslab_class_get_alloc and metaslab_class_get_space considers parity disk space as well which will be different from change #89. This info will be documented for internal reference.