[Drbd-dev] DRBD-8: Update to handle I/O failures during resync

[Graham, Simon]

 <<drbd-resync-err.patch>> Attached is a patch that adds support for
handling I/O errors during resync
as previously discussed.

Changes are:

1. New fields are added to the mdev and bm_extent structs to count the
number of 
   failed requests during resync - the field in bm_extent is used to
determine 
   when we are finished with the extent at which time it is zeroed so
that next
   time we process the same extent, we'll try to resync everything
again.
2. Resync is considered done when the remaining count of block to sync
is <= the 
   count of failed requests.
3. When a failure occurs, a new routine, drbd_rs_failed_io() is called
to handle
   this - it takes care of incrementing the failed counts and
determining if the
   resync operation is now done. This is called when a NegRSDreply is
sent or
   received and also when a NegAck is received for a resync operation
(this is
   necessary to ensure that the SyncSource correctly knows when the
resync is done).
4. When a resync operation completes and there have been failure, the
new code
   does NOT rotate the UUIDs - I felt this was correct because the
resync really
   hasn't completed in this case (also, I found that if I did not do
this, the
   code would end up resyncing the entire disk next time!)

I did not introduce any new states for this - let me know if you think
there needs
to be one; with this patch, we simply end up Connected and Inconsistent.

Simon

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