xfs: refactor xlog_recover_process_iunlinks()

For upcoming changes to the way inode unlinked list processing is
done, the structure of recovery needs to change slightly. We also
really need to untangle the messy error handling in list recovery
so that actions like emptying the bucket on inode lookup failure
are associated with the bucket list walk failing, not failing
to look up the inode.

Refactor the recovery code now to keep the re-organisation seperate
to the algorithm changes.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>