exofs: Multi-device mirror support

Submitted for review an exofs support for multiple osd targets in a
mirror configuration.

Depending on pending scsi/osd patches I would like to push these
in the next exofs push-out. I've been playing with some version of
these patches for a while now, testing both as stand alone, as well
as, exported to pnfs-objects clients. They are now quite stable, though
a bit conservative in regard to what more can be done even in simple
mirror configuration.

The user-mode mkfs.exofs supports almost the full range of pnfs-objects
fixtures, but the Kernel driver will only mount a mirror arrangement.
Once these go through we are planning support for raids 0, 4, 5, and 6
over mirrors over device groups. See second patch's commit log for some
more explanations.

The new user-mode utilities as well as these patches can be found at:
  http://git.open-osd.org/gitweb.cgi?p=open-osd.git;a=summary

I will push these patches to linux-next once all comments subside.

List of patches:
[PATCH 1/3] exofs: move osd.c to ios.c
[PATCH 2/3] exofs: Move all operations to an io_engine
[PATCH 3/3] exofs: Multi-device mirror support

Thanks for review
Boaz

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