xfs: make the start pointer passed to btree update_lastrec functions const

This btree function is called when updating a record in the rightmost
block of a btree so that we can update the AGF's longest free extent
length field.  Neither parameter is supposed to be updated, so mark them
both const.

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