xfs: make the key parameters to all btree query range functions const

xfs: make the key parameters to all btree query range functions const

Range query functions are not supposed to modify the query keys that are
being passed in, so mark them all const.

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