Update ZFS metaslab code from OpenSolaris.

Update ZFS metaslab code from OpenSolaris.
This provides a noticeable write speedup, especially on pools with
less than 30% of free space.

Detailed information (OpenSolaris onnv changesets and Bug IDs):

11146:7e58f40bcb1c
6826241	Sync write IOPS drops dramatically during TXG sync
6869229	zfs should switch to shiny new metaslabs more frequently

11728:59fdb3b856f6
6918420	zdb -m has issues printing metaslab statistics

12047:7c1fcc8419ca
6917066	zfs block picking can be improved

Approved by:	delphij (mentor)
Obtained from:	OpenSolaris (Bug ID 6826241, 6869229, 6918420, 6917066)
MFC after:	2 weeks