[XFS] split up xlog_recover_process_iunlinks

Split out the body of the main loop into a separate helper to make the
code readable.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dave Chinner <david@fromorbit.com>
Signed-off-by: Niv Sardi <xaiki@sgi.com>