xfs: pass the iunlink item to the xfs_iunlink_update_dinode trace point

So that decoding is only done when tracing is actually enabled and the
call site look a lot neater.

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