devices are imbalanced zfs will lots of CPU searching for space on devices
which tend to be pretty full. It should instead fail quickly on the full
devices and move onto devices which have more availability.
New loader tunable: vfs.zfs.mg_alloc_failures (min = 8)
Illumos-gate changeset: 13379:4df42cc92254
Obtained from: Illumos (Bug #1051)
MFC after: 2 weeks