Patch propisal: device opening and creation speedup

[Martin Matuska]

Hi, I would like to propose the following two upstream changesets for
import:

OpenSolaris revision 9846:6527c7b4a92e
Bug ID 6566744: vdev_open() should be done in parallel

OpenSolaris revision 10588:dc03f981ea18 (partial)
- vdev.c change for zpools on top of zvol devices

http://people.freebsd.org/~mm/patches/zfs/head-9846-10588p.patch

The code is identical to state of V28 and produces a noticeable speedup of vdev_open() and vdev_create() on systems with many devices (e.g. storage servers - FreeNAS etc.). It would be good to have this in 8.2-RELEASE as well.