xfs: document what LARP means

Christoph requested a blurb somewhere explaining exactly what LARP
means.  I don't know of a good place other than the source code (debug
knobs aren't covered in Documentation/), so here it is.

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