ZFS patch proposal: offlining log devices

[Martin Matuska]

The handling of log devices prior to pool version 19 is quite
problematic. They cannot be removed nor offlined.

I want to propose a patch against head and later MFC that imports the
following OpenSolaris revision, that enables offlining of log devices:

9701:cc5b64682e64
6803605 should be able to offline log devices
6726045 vdev_deflate_ratio is not set when offlining a log device
6599442 zpool import has faults in the display

http://mfsbsd.vx.sk/zfs/head-9701.patch

I did successfully test the following:
- offlining a log device (ZIL is written to disk and device offlined)
- importing the pool without offlined log devices (UNAVAIL with saved
zpool.cache)

Next I would also like to propose this additional patch:

http://mfsbsd.vx.sk/zfs/head-9725.patch

9725:0bf7402e8022
6843014 ZFS B_FAILFAST handling is broken

Both patches reduce diff against v28.

Thank you,
mm