xfs: make the keys and records passed to btree inorder functions const

xfs: make the keys and records passed to btree inorder functions const

The inorder functions are simple predicates, which means that they don't
modify the parameters.  Mark them all const.

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