Illumos 4754 4755

4754 io issued to near-full luns even after setting noalloc threshold
4755 mg_alloc_failures is no longer needed
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Adam Leventhal <ahl@delphix.com>
Reviewed by: Dan McDonald <danmcd@omniti.com>
Approved by: Dan McDonald <danmcd@omniti.com>
Ported by: Tim Chase <tim@chase2k.com>