[XFS] fix error handling in xlog_recover_process_one_iunlink

If we fail after xfs_iget we have to drop the reference count, spotted
by Dave Chinner.  Also remove some useless asserts and stop trying to
deal with di_mode == 0 inodes because never gets those without passing
the IGET_CREATE flag to xfs_iget.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Eric Sandeen <sandeen@sandeen.net>
Signed-off-by: Niv Sardi <xaiki@sgi.com>