MFC 224177:

MFC 224177:
ZFS tries to allocate blocks evenly across all devices. This means when
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)

git-svn-id: svn://svn.freebsd.org/base/stable/8@224565 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f